I bought a Mac Pro Quad this month and I'm not disappointed, even though I'm sure I overpaid for what it is. Here's why: When I spec out a 27" iMac Quad the way I would want it, with the 2.8 GHz i7 and bump to 8G of RAM, it costs $2400 before taxes. That's alot of dough to spend on a computer that is fixed, that you can't upgrade, and that is tied to a screen. When I spec out a Quad Mac Pro, with an upgrade to the ATI Radeon HD 4870 graphics card and bumping the RAM up to 6GB, it costs $2850 before taxes. That's a difference of $450, for upgrade-ability, expandability, more power, and a choice of screen. Even without all the other goodies already mentioned that you get with a Mac Pro over the iMac, the differences I've listed here would be worth $450 to me. Especially over the 5 years or more I will own the Mac Pro.
Now, if you're someone who is in love with the iMac screen (Apple's new glossy glass-like screens), then I can see wanting the iMac. Myself, I don't like them. Too much reflection. I like a matte screen with no glass in front of it. That's the main reason I went Mac Pro: choice of screen. So for me, you'd have to pay me more than $450 to get me to use one of those glossy screens for five years, and then have an obsolete computer.
The iMac Quad isn't priced that far under the current Mac Pro Quad if you look at what you're getting. Of course, if you're one of those people who like those new iMac screens, and don't already own a screen (i.e., you aren't upgrading from an older tower where you already invested in a nice screen) then I could see how the value equation changes when you look at the iMac.
My recommendation: If you want to get out of the consumer computer cycle, buy a nice screen that will last you ten years, then choose your computer based on its merits as well.