To tell you the truth, apple's current approach of form over function at the expense of cooling (MBPs reaching 90ºC; "questionable" cooling on the new iMac) and the way they may end up implementing thunderbolt, which really isn't of that much use when you have pci-e slots, on that "something really great coming out in 2013", is really putting me off.
Now thunderbolt may come in three flavours, you either have it in a 1155 cpu with it's built in gpu coupled with the thunderbolt controller for both thunderbolt+video with a slot for a video card. You can also have it in the form of a LGA2011 board with a soldered gpu (asus has a test board like that). Pci-e slots if there are lanes left and more HDD slots to not make it look like a headless iMac.
All these options are obvious deal breakers because they go against the reason I picked a Mac Pro and in the past, PowerMacs.
Now if apple still wants to moronly put thunderbolt in that "something really great coming next year", they could, simply, put a displayport IN connector in the motherboard that is coupled to the thunderbolt controller, should you desire to have video+thunderbolt, for, let's see, the thunderbolt display. Of course such option would come at the expense of pci-e slots, but it'd be using a normal pci-e gpu. All you'd have and that is if you wanted thunderbolt+video, would be a display port cable coming from the gpu connected to that displayport IN connector I mentioned. But i guess this approach is just wishful thinking at this point.
With this in mind a new Mac Pro revision would just require, besides the obvious up to date xeons, four 8-pin connectors for modern graphics/compute cards and seriously, fan inlet filters. But I see apple neither following this nor what I've mentioned in my previous paragraph. Otherwise a new revision wouldn't have been delayed. And seriously, taking "that" Tim Cook mail as legit, the Mac Pro is EOL and what is coming next year is an entire new product that may not suite the markets the Mac Pro used to target, the pro-video/audio and the scientific-academic markets (well I guess this one died with the xserve and was buried when xgrid got axed).
To finish my wall of text, on light of what I've mentioned, I'm seriously considering a PC workstation running Fedora, not just because my workflow of Vim, Matlab, Mathematica, TeX, LibreOffice allows me to, but I also have a strong distaste for Micro$oft's quasi-facistic licensing (and Windows Wait).