Slight niggle here: Please, please, PLEASE keep this discussion limited to the Mac Pro. If someone wants to make a thread for the xMac like this, I would be happy to see one, but don't post any xMac ramblings here, please. Thank you in advance.
Updated to reflect posts as of 1/19/2009; 6:09 A.M. Now with epic mockup action!
This thread exists because I acknowledge that, while I have been very good at getting future specs correct in the past, I'm a little out of my league here. I don't know much about Intel's workstation hardware. I just want to learn more, is all! It's never a bad thing to learn. Also, it's nice to have a repository of information. Remember Multimedia's huge thread? I just hope that I don't wind up banned because of this one...

This first post will be edited as we gain more and more clarified information. The newest additions to general information will be in red. For everything underlined, I would personally like clarification on my points and information based on what everyone thinks will happen to said point, either for or against what I have said.
Points yet to be decided upon:
Gainestown production status: Unknown, presumed not begun
Release Date: H1 2009, before Snow Leopard (WWDC 2009 inclusive)
Base Price: $2,999
Ethernet: Dual gigabit or dual 10G
FireWire: two 800, two 3200, or two 400/two 800 (unlikely)
Hardware RAID: Does Tylersburg support it?
Fiber card: Will a better fiber card stop us from getting 10G Ethernet?
2TB HDD: Only possible if the WD HDDs are released before the Mac Pro... will they be?
SSD: What is the capacity of available 3.5" SSDs?
AirPort Extreme: Will it be standard? Apple had a rationale last time for not making it standard.
Hardware Specifications:
2.66, 2.8, 2.93, and 3.2GHz 8-core Gainestown Xeon processors
2 risers with 12 slots for 1333MHz ECC DDR3 RAM. It will not be FB-DIMMs. They must be added in sets of three DIMMs at once. Up to 96GB of RAM will be physically possible at release.
The ODDs will change to SATA
The three graphics options will be: up to four
GeForce 9600GT or
Radeon 4670, the
GTX 260, and the
nVidia Quadro FX 4800 or 5800
500GB HDD standard, with options for 750, 1TB, and 2TB*, as well as an SSD option
(potential size?) and a 300, 450, or 600GB SAS option.
I/O:
PCIe 2.0 expansion: two 16x and two 4x, or four 16x
(here's something I just thought of: two double-wide 16x?)
Bluetooth 2.1 EDR
5 (minimum) USB 2.0
Optical audio I/O
Analog audio I/O
Things we won't see:
USB 3.0 (2010, people)
WiMax (just making sure you're paying attention)
Blu-ray (see below)
Narrative:
SO! It's almost that time of the cycle again! We're getting closer to the beloved 518 days since a Mac Pro update, and boy, oh, boy, are things heating up! Or at least, they SHOULD have been. In fact, Gainestown was scheduled for a November 2008 production back in the middle of last year, but, along with everything else in the Nehalem line (except for Core i7...dorks...), has been pushed back. We have been hearing "Q1 production" for a while, but haven't heard anything recently.
The processor, Gainestown:
Gainestown is the two-processor variant of the Nehalem Xeon. The two most likely candidates for the Nehalem Mac Pro are the W5580 and the X5570. It will also include either
(both?) the X5560 or the X5550. These four processors clock in at 3.2, 2.93, 2.8, and 2.66GHz, respectively. In 1,000 count lots, these processors cost $3,200, $2,772, $2,344, and $1,916, respectively.
That's a
lot of money for the upper three, folks. I'll get into why those prices are significant later.
Assuming the 2.8GHz chip as the standard configuration, the base model price is expected to jump to around $3,000. Assuming the 2.66GHz chip as a BTO option to make it cheaper, the lowest price for the Mac Pro would become around $2,500.
The chipset, Tylersburg:
Gainestown goes in the Tylersburg chipset. Tylersburg supports two I/O hubs, meaning that we could see up to 4 PCIe 2.0 16x slots and 2 PCIe 2.0 4x slots. That's cool. I, personally, refuse to believe that Apple would do that, for various reasons known only to them. I think that we might see either two 16x and two 4x or four 16x.
Now the little things:
AirPort Extreme (802.11 a/b/g/n). Will it remain optional? I thought the reasoning behind this was that some businesses didn't want Wi-Fi in their computers, but I could be way off. It's been QUITE a while since I've heard any rationale on leaving it out.
FireWire: The current Mac Pro has two 400 and two 800. Here are two actually plausible scenarios: two 800 only or two
3200 (does Tylersburg support it?) I do not believe that Apple will retain the FireWire 400, given they removed it from their other pro product, the MacBook Pro, and failed to add it to the 17" model after three months' time to see the response.
GPUs: Discussion on what GPUs will be available is open, as well, but what about Mini DisplayPort? I believe that Apple will have all models include two Mini DisplayPort ports, and include IN THE BOX two Mini DisplayPort to single-link DVI adapters (making the dual-link DVI adapter a BTO thing at, perhaps, a slight discount?)
Blu-ray: Ugh. Here it is. Apple will be forced to add Blu-ray around 2015 to remain competitive with video media.
400GB Blu-ray disks will be in production in 2012-3. Whats the point? you ask. Super Hi-Vision. Super Hi-Vision is the next video format. Its 7680x4320. Thats 4320p. Super Hi-Vision in MPEG-2 is 600Mb/s.
U.S. Internet speeds WILL NOT keep up with this. Heck, most of us cant get 1080p in anywhere near a decent amount of time. To top it off, many of the major ISPs are putting monthly caps on bandwidth and time limits on the maximum advertised speed of their service.
We wont be able to download 4320p movies in iTunes because our ISPs will give us the freaking shaft. Itll be days to the download, even in 2015. Well all mostly have the bandwidth for very quick 1080p movie downloads, so iTunes will be able to offer that, but itll be the same situation in the future as it is now: Instead of people having 1920x1080 TVs with their SD iTunes movies playing on them through an Apple TV, people will have 7680x4320 TVs with 1080p movies playing on them through an Apple TV.
Where do I get my date for Super Hi-Vision adoption? Well, Japan will start 4320p television broadcasts in 2012, and the 400GB Blu-ray disks go into production in 2013
Television manufacturers are starting to make half-Super Hi-Vision TVs (4096x2160) on the highest-end already.
Redesign talk:
I have never been a proponent of a Mac Pro case redesign. Well, I was, but I was young and naïve. Now, I see no case redesign for three reasons:
1. WHY. It STILL looks amazing; the cheese grater is timeless.
2. The Mac Pro's audience generally tucks the tower out of the way after purchase, anyway. As much as it could be, it isn't a "show" computer.
3. Most importantly... what would it be changed to? For the past year, I've been trying to come up with a new design, and I couldn't do it because I just have no idea how to make it look better... because there isn't a lot that you can do to make it look better.
Explanation Section:
Release date: Based on the pushing back of production dates and the build time required thereafter, late spring or WWDC 2009 are good dates for release. Coincidentally, WWDC 2009 takes place exactly 518 days after the release of the Penryn Mac Pro. For those of you following my "518 days" complaint, you know what I'm talking about.

The release date is independent of Snow Leopard's release; they have nothing to do with one another.
In the past, the Mac Pro has been released from 6-13 weeks after production begins.
$2,999: Apple should be getting a better price than that of the 1,000 lot price, because they'll be ordering more. However, the per-consumer price of Harpertown was the same as the 1,000 lot price for those chips so we'll see something very similar with Gainestown.
SATA ODDs: Based on the change for the MacBook line. Besides, even though ODDs don't get anywhere close to saturating the SATA transfer rate, ATA is old, man! Tylersburg supports six SATA ports, so that's four for HDDs, and then the other two (which exist on the current logic board hidden behind the fanssome people use them) can be used for the ODDs.
2TB HDD: Based on the fiasco associated with Seagate's 1.5TB drives and based on the time it takes to actually get this computer out to us, Apple might have 2TB 3.5" drives available to take advantage of as a stupidly expensive BTO option.
More to be added: your input is greatly appreciated. Discussion, ho!
**Contributors:
Umbongo: Tylersburg I/O clarification, GPU suggestions, real-world RAM limits, and better thread title than my unposted original
RobLS: reminding me about AirPort
Mattww: reminding me to add RAM clock speed and ECC
Greenhoe: reminding me about Mini DisplayPort on the GPUs.
rylin: for giving me the color timestamp idea
Eidorian: Radeon 4670 correction
J the Ninja: convincing me that they'll only use two risers. It makes sense.
iMacmatician: multiple lowest-end GPU reminder, CPU speculation, and Blu-ray placement fix
bradleykavin: I needed to add downtime between production and release