The Mac Pro is worth its high asking price...any similar Dell Xeon-based machine will cost pretty much the same, and I'd take the Mac Pro over the Dell due to its ability to run Windows and OS X.
It would be nice if Apple came out with a Core 2 Duo-based tower at a lower price point than the Mac Pro, but that won't happen because the iMac fills that slot right now.
The Mac Pro is very good value, I agree. It is scarcely more expensive than a Macbook Pro (a Pro laptop with a grainy screen... go figure, but that's a discussion for another day). But the good question is, is it worth paying 4 grand just for the ability to upgrade your video card?
What if I don't agree with Apple's choice of video cards in the stock configurations for the iMacs?