Well, I don't have a Mac Pro (yet)... Probably will buy when the quad-core CPUs arrive. But I currently have two Power Mac G5 Quad systems, one with 4GB, the other with 8GB. ...And several PeeCee systems running Win64 and/or Linux with 4GB.
I'm a 3D artist and video guy and my work is all done in HD resolutions these days. I find the 4GB configs work just great, but I have occasionally hit the 4GB wall on some intense projects. 8GB is more than adequate for my work right now -- not that I couldn't use it, but the software just doesn't allow for it yet. Most applications are still limited in the 32bit realm, which means no more than 2GB per app process, but with Lightwave, Modo, Photoshop and one or two other things open at once, I can populate 4GB pretty easily. I did find 8GB limiting on an architectural viz project I did a couple months ago, but that was an extreme case not typical of my usual workflow.
I have actually been seriously considering upgrading my G5 quad at home (the one I mentioned with 4GB) to 8GB so it matches the one at the office. Lately I have been working from home a lot more and the 4GB ceiling is right there and I know I'm going to bump into it sooner than later with a few things I'm working on.