Stand corrected on the dates for Mac Pro/G5, my bad. It's not like Leopard is suddenly going to go up in a ball of smoke come WWDC 09.
I guess definition of "current" can vary from place to place - "Top of the line" is by definition something that lasts for only ~6 months or so. If you got in just before a major change, then the "making less powerful in comparison" of your kit through the newest update to a line can be bigger.
If Jobs says the architecture will be supported, he doesn't have to mean supported by the latest OS. As previously stated - If Snow Leopard is Leopard at the core, with improvements in certain areas benefiting the recent models and upcoming models, this doesn't stop the updates and bug fixes and improvements from filtering to Leopard.
For PPC users - if it's easy for app developers to quickly make PPC versions, then why not? If there is an actual market (there certainly is now) then they'll cater for it if it's profitable. Yes, supporting one platform is easier, but that's not to say we don't see applications on more than platform these days, is it?
I don't think you can logically move through rationalisation all the way to saying PPC won't benefit from Snow Leopard (the implicit meaning being that it won't benefit at all). If PPCs are on Leopard, and Leopard gets benefits from improvements in Snow Leopard, then PPCs get indirect benefits.
Mythical mid tower - yup, another debate! Know the feelings about wanting something they haven't/are not/may not make.
Apple's kept the priority to concerns of it's customers. I think just by sheer Mac sales, they might actually be prioritising a big chunk of Mac owners through Snow Leopard, by the time it's out. Those future Macs exist between now and WWDC, and beyond. Some of the tech they're pushing - it's only in it's infancy/not been seen outside of a few press releases. Some is more open, but we don't know what Apple's doing with it. It's Apple's first big 64-bit consumer OS go at addressing parallelism - addressing getting the OS to do the work in sorting out how to most efficiently/powerfully handle multi-core, multi-processor Macs.
I don't think I've seen much arguments on this thread as to why any PPC machine should actually benefit grandly from Snow Leopard - is it more a case of "I paid for this when it was top of the line, which wasn't that long ago, and it should still be at the forefront, so I should by right to be able to run the most recent Mac OS" ?
(Cartman saying "respect mah authoritay springs to mind).
We've got a way to go yet. You can't really roll back or reverse a product becoming inferior to it's updated version. It can be a smooth feeling, and it can be a stochastic feeling. Processor bumps on a certain line can show a smooth upgrading - you had a 2.0, now there's 2.5, oh look there's 2.8, 3.0, 3.2 and so on, month by month. I'd imagine Nehalem is going to be quite a large shift towards making everything quite a bit more inferior power wise.
It's the sharp side of "Software sells systems". It also may bring GPGPU, multi-core multi-processor benefits, Grand Central, OpenCL, and coming alongside seeing javascript getting enough speed to be a much bigger player, HTML 5 at some point, all sorts.
When you say giving priority to the concerns of its customers - define when those were customers? Just today? yesterday? Last month, week, year? 2, 3, 4, 5 years ago? It is brutal. You can see many upgrades making your Apple product in your hand, at your desk, just that little more obsolete every product launch. The latest Touch now having Nike+. The latest nano now having accelerometer. The latest laptop having a multi-touch trackpad, the latest laptop/desktop soon to be shifting to different mainboards, Nehalem based CPUs, possibly having other hardware as well.
If people really want to be at the cutting edge, there are options - you can sell old kit, buy new kit/refurb new kit. PPC owners seem to be going to feel the brunt of a quite big Apple leap forward. Is the main part of the chagrin due to the fact that if they really wanted, they could bring Snow Leopard to PPCs? it's a painful thing second guessing Apple sometimes. Also painful if you think of what they could do, versus what they do in reality. Not raising expectations overly so. MMS. Bluetooth. Disk access on iPhone. Accessible batteries, CPU, Memory. Swappable/upgradable components. Apple does wield this as a way to get you to buy new hardware.
All of this isn't to in any way demean a PPC user's position - I and anyone else can feel for them. Most people have had a similar feeling about their older Apple hardware. The price of progress, deemed by Apple.
But has Apple really suddenly lost making a commitment to its existing products?

Your level to which you can upgrade your software/OS is bound in part by your hardware, which is to an extent fixed at the time you bought it. Apple i'd imagine will have commitment to Leopard - why wouldn't it? It's currently it's most recent OS, a version is on it's iPhone, the Touch, a significant chunk of its users are using it, and chunk of those won't be able to shift up to Snow Leopard.
Depends what you mean by "making a commitment to its existing products".
It doesn't seem Apple wants to shackle itself to making a commitment saying - existing products should forward compatible. Back compatible yes, forward, not so much.
Hope it doesn't come across as a rant

Apple would I imagine be working it's behinds off to get this coming through. It'll filter improvements to those on Leopard, Snow Leopard, those with iPhones, those with Touchs. And if you're lucky enough to be looking to buy in a couple of months, you'll get the benefits of some big jumps in hardware, that'll (eventually!) get incorporated into Macs to a degree. Heck - if we're looking far enough ahead to Snow Leopard, anyone with a 3G iPhone should be thinking about what the 3rd generation/v3 iPhone will be like. Because it's likely to be impressive enough to "make obsolete" (which is usually used to mean make inferior, rather than strictly mean "no longer in use". Heck - With Apple being selective about both the areas it works in, and the features it provides, it always gives them lots of scope to inferiorise through making superior next generation kit. In the current iPhones case - GPS, camera, video, VoIP, headset, Nike +, the graphics chip, the CPU, the memory size... they could all get bumped, and prove Stephen Fry's prediction right.
Edit: Having a look at a link in another thread with respect to delays for Nehalem - how'll that pan out for Apple? Will they keep working on SL and release it later?