I'm actually a bit surprised at how much people complain about Mac Pros being refreshed infrequently.
The Mac Pro is Apple's machine with the LONGEST useful life of them all! I'm still using my 2006 Mac Pro, and it STILL feels fast even today! Anyone with the 2010 Mac Pro should still be cruising with a blisteringly fast system and not need to upgrade anytime soon.
Meanwhile, my former 2006 Macbook Pro (I sold it) was quite slow and very limited, especially by its maximum RAM capacity.
Mac Pros are investments, meant to be used for several years. Unless you're absurdly rich and must always have the latest gear, this should be common sense.
I'll probably be buying a new Mac Pro at the next refresh (five years into the ownership of my current one). How many other computers can you get five productive years out of before they start feeling slow? I'm a bit annoyed that I'll be forced to move to OS X Lion, but I'm sure they'll get some of its issues resolved by then.