I think to drop the Mac Pro at this moment in time would be incredibly short sighted, thunderbolt just isn't mature enough to replace PCIe, in the future it may be, and the big workstation may very well become obsolete for nearly everyone.
I still found the dropping of the xServe line short sighted, I'd love to have some hard data as to how many people changed their upgrade policy in education/enterprise due to them dropping the xServe. After all the xServe was generally purchased to serve a number of mac desktops or laptops, and for some the mini server, or mac pro server was not up to scratch. So how many macs have not been purchased due to the dropping of the xServe, instead replace by windows or linux servers that maintain non apple hardware. Many of these macs that have, or are being replaced will be in education, where kids will learn how to use computers, when I was in college I found the mac the best (and at the time only option), but if I'd have been taught on windows I may be singing a different tune. If the next generation of prospective apple customers never get a chance to use a mac, they may never choose to buy one in the future.
Likewise, every studio I know, and every post production complex are built around macs, the mac pros run the Pro Tools HD rigs, they run the film editing software, If the pro line is cut short these places may very well migrate to windows, and at some point the receptionists shiny iMac may be replaced by a swanky Vaio, the voice directors iPad might get replaced by an android or windows tablet, so there is conformity across the complex. Just like once Dr. Dre, Metallica, Beastie Boys, Marilyn Manson or whoever is in vogue today start cutting their albums only on Windows, and the "cool" filmmakers start doing the same, part of the apple cool factor may dissipate. Apple has been riding the cool factor for many a year, wether they like it or not, people still see apple as the creative persons computer, even if they have become more generic all purpose computing devices.
If they drop the line in a few years once there are easy and workable solutions to do a lot of the stuff that currently only the Mac Pro can do you probably won't hear me argue, maybe just a few incoherent grumbles, after all IMHO the Mac Pro is apples best ever designed computer, it is a perfect match of form and function, from a company who often have been guilty of form over function, the Cube is a close second
i'm typing this on my i7 iMac, after years of swearing to never buy an all in one again, but the iMac line has come a long way, and suits my needs now nearly perfectly, I do miss the PowerMacs drive bays (or the ones I had in the hackintosh I was using for about 18 months). Even at work we only have the one Mac Pro, but that's because currently it is needed for the Pro Tools HD rig, everything else is iMacs or MBP, or the mini server.