The New MacPro: Some Informed Guesswork

At this point, we're all guessing, but this article on Ars Technica contains more informed guessing than most of the breathless rumor sites.
Chris and Trish Meyer

For high-end Mac users who have been curious what the rumored new MacPro might contains, Ars Technica recently published a well-researched and -explained bit of speculation. They spent some time going into the the evolution of various Intel processors, including issues with feature sets and silicon yields (they're putting their money on the recently-released Sandy Bridge-EP-based Xeon E5s), as well as the problem of supporting Thunderbolt with DisplayPort connectivity on a machine that contains an interchangeable plug-in video card rather than circuitry built into the motherboard (look for a jumper cable - as well as cards that have an internal DisplayPort connector). As I mentioned above, we're all guessing, but this seems to be more informed than most.