It's because there is nothing to upgrade to.
Kind of. Nehalem and all that is out already, but Apple is always a half generation behind in the hardware category. Look at GPUs
No, the chips for the Mac Pro AREN'T out already.
Ah - then quite possibly the reason for at least the Pro's stagnation isn't entirely down to Apple.
I'm almost tempted to label Apple 'fickle' with some of their products. When there's something interesting they can do, they're all over it. It seems, perhaps, there's nothing compelling out there to make them give the Pro some love. Unfortunately for the purchaser, they don't tend to drop the price on tech as it ages, preferring to discount only after a model has been superseded.
Again, I'm not terribly up-to-date with Intel's plans. However, I get the feeling that high-performance 'consumer' parts tend to develop quicker than their equivalent workstation/server parts (upon which the Pro is based). So, towards the end of the life of the Pro's architecture, the Intel 'consumer' range has almost met or surpassed its performance. This leaves the workstation class machines looking unattractive until the big leap which once again justifies their classification.
Does that make any sense (or, indeed, bear any relationship to reality)?
edit:
A few thoughts on GPUs in the Pro.
As I understand it, many of those who'll be pushing the Pro to its (computational) limits are more interested in sheer bit-crunching ability rather than shader models and pipeline counts. Is it correct that Photoshop, Final Cut, Shake and all that jazz really wants CPU and architectural horsepower? In these situations, a new machine has the ability to save sizeable amounts of time each and every working day. When a pro user's waiting for something to process, they're not creating anything.
I can sympathise to an extent. Coding on a slow machine is a nightmare (especially when your toolkit - Visual Studio, in my case, is on the 'chubby' side). If I'm waiting 5 minutes for a compile instead of 10.... that's significant. If I can run two or more virtual machines simultaneously with great performance -- that's huge.
In these situations, I could have Intel GMA GPU and not care in the slightest.
Of course, Snow Leopard has the potential to turn all this on its head, with co-operation from Adobe and other vendors.