There are two major "flavors" of E5. The 1600 and 2600 series.
http://www.cpu-world.com/news_2011/2011050402_Intel_Xeon_E5-1600_and_E5-2600_processor_details.html
The 'single package' Mac Pro can be implemented with a 1600 and the dual package with a 2600. Both are E5's and have 40 PCI-e v3.0 lanes per package. The descendant of the Xeon 3500 and 3600 series is the E5 1600 series not the E3 models.
Aren't Ivy Bridge E3's due in the Q2 2012?
Two major problems with that.
1. The E3 package only has 20 PCI-e lanes. Even if the the Ivy Bridge version matches the twice as fast v3.0 implementation it is still 20 lanes short in bandwidth. That is a substantial bandwidth loss. If all you need is one 16x card perhaps that isn't a loss, but for a 4 slot machine that is a major drop in value ( if 4 slot present they are highly switched/shared. )
2. Q2 is awfully late. If Apple doesn't launch a new Mac Pro by Feb the "doom and gloom" rumors are going to spin out of control. There are lots of folks waiting on the sidelines right now. However, those folks have work to do. At some point people are going to buy whats available. The other PC vendors are going to launch E5 boxes. Apple passing up E5 to wait another 2-3 months to launch a box with substantially less bandwidth ..... don't see it at all.
3. The "core i7" version of the E3 will also be out in Q2 and the iMac will get it. Again why? hold up the Mac Pro for an iMac part? Cost isn't a major issue since the 1600 parts are likely in substantially the same range at the entry levels .
http://www.cpu-world.com/news_2011/..._and_prices_for_Xeon_E3-1200_series_CPUs.html
http://www.cpu-world.com/news_2011/2011091901_Prices_of_Intel_Xeon_E5-1600_series_CPUs.html
E3 1225 --> $280 ( 3.1 GHz & 4 cores 8MB L3 cache )
E3 1235 --> $336 ( 3.2 GHz & 4 cores 8MB L3 cache )
E5 1620 --> $294 ( 3.6 GHz & 4 cores 10MB L3 cache )
There is not substantive difference in pricing and if just chasing clock speed the E5 is a better value.
The E5 1650 jumps to $583 but also get two more cores (and are drifting into 'apples and oranges' comparisons ). However, the $583 1650 should allow the 6 core Mac Pro model to shift from the current $3699 price point down to $2899. That will be approximately "$1,000" drop that many folks a grumbling for.
Are they going to offer a single-processor E3 and a dual-processor E5?
The only reason to do a E3 box would be to come out with a shorter , one 16x PCI-e slot model that was priced at the $2K price border shared with the iMac. They could but that would not need to remove the E5 options being sold at higher ( above $2,300 ) price points.
Apple going to introduce at xMac box that carves into the sweet spot of the iMac price range ? .... keep drinking that kool-aid .