The current Mac Pro, namely the 2009/2010/2012 model, is fast enough.
And yet every single market competitor to the Mac Pro came out with a new versions with native SATA III, USB 3.0 (discrete controller) , and PCI-e v3.0 support (along with Sandy Bridge upgrades).
I doubt they really even meant for the 2012 model to be released until they saw all the excitement and hope for a new model prior to the keynote last year.
Apple has a policy about talking about future products. Unless they released a Mac Pro they really can't talk about the Mac Pro. The 2012 release was far more a statement that the Mac Pro was not dead. That was the for more pressing communications gap at the time. This still lingers even after statements they are working on something.
The hysteria about sweeping all new Mac line-up before WWDC just because there wasn't going to be an iPhone? As long as folks were whipped up that all Apple cared about.... weaving a Mac Pro into that was a non issue. There wasn't even a press release. After a pitch fork mob erupted over the release they even took the "new" sticker off of it in the Online store.
There is no good reason not to have released a Sandy Bridge upgrade. Absolutely no sane rational reason to leave the video card trapped in a time if this was a long term planned release.
I feel that Apple decided to skip the Sandy Bridge E series because the advantages really were not that great, with marginal performance increases.
In the era of 6.0 Gbps SSDs it is extremely competitive to be stuck with a 3.0Gps SATA implementation.... not. Even the Mac Mini isn't that blocked with legacy infrastructure.
Apple didn't release anything because they did not (and still don't) have anything. It isn't the new technology isn't incrementally better in a significant way; it is. It is likely because there is nothing to ship.
I think sometime late this year, as in after June/July, we'll see the new model based around Ivy Bridge - and decent performance increases - enough to warrant the upgrade in the first place!
The substantive improvement in terms of x86 cores performance between Sandy Bridge and Ivy Bridge is marginal. In most contexts, most performance boost will come from simply higher nominal clock speeds. The IOHub support chips for the Xeon E5 class offerings is very likely to be exactly the same ( meaning exact same board with firmware bump can be used for both .... just like last 2-3 Mac Pro class tick-tock iterations. )
There deep confusion between Ivy Bridge the microarchitecture and the content of the product packaging done with on-die intergration.
Here are the Ivy Bridge deltas.
http://en.wikipedia.org/wiki/Ivy_Bridge_(microarchitecture)#Ivy.C2.A0Bridge_processor_features
1. PCI Express 3.0 support.
[ Buzzz not really accurate because Sandy Bridge Xeon E5 class has this. No advantage Ivy Bridge. ]
2. Max CPU multiplier of 63 (57 for Sandy Bridge)
[ likely not of much significant in Xeon E5 class processors with more
conservatively fixed multipliers. ]
3. RAM support up to 2800 MT/s in 200 MHz increments
[ 1600 MHz DDR3 RAM ... buzz again already present in Sandy Bridge Xeon E5 class products. ]
4. The built-in GPU ...
[ buzz no built-in GPU hardware in Sandy or Ivy Bridge Xeon E5 class products. A complete non system issue if just use embedded discrete GPU solution. ]
5. DDR3L and Configurable TDP for mobile processors.
[ Yep that is going to make a significant difference in a desktop workstation context .... NOT ! ]
6. Multiple 4K video playback.
[ Buzz see number 4 , same issue. ]
7. Intel Quick Sync Video
[ Buzz see number 4 , same issue ]
8. PCIe implementation incompatible with most SAS 6G and USB3 cards,
[ Not sure how this would be benefit. As Apple has stopped selling SAS solutions. None issue. ]
9. Up to three displays are supported
[ Buzz see number 4 , same issue]
Ivy Bridge Xeon E5 will top out at 10 x86 cores instead of 8 in a dual package set up (using 2600 series). That has zero impact except at the upper range $5,000+ offering. Ivy Bridge Xeon E5 1600 may start out at 6 cores. (doubtful. likely the same 4 , 6 , 6 set of offerings in the price zone Apple is likely to go with if used Sandy Bridge offerings. There may be an 8 core E5 1600 v2 but it likely will carry a hefty price premium to pad Intel's margins. )
Ivy Bridge will be clocked incrementally higher so it would be an improvement but there is absolutely nothing indicating there Sandy Bridge is hugely deficient in some way. At the desktop offering level the vast majority of the improvement was allotted to graphics not x86 improvements. There was also a new IOHub update. Neither of those things are coming to the likely Mac Pro class options.