An article on virtualization (e.g., running Windows and MacOS simultaneously on an Intel-based Mac) is in today's Wired News.
Parallels' Rudolph said dual-booting systems are not in tune with the way people work. Having to shut down OS X to boot into Windows wastes time, he said, adding that a Mac running Parallels' virtualization software is almost as fast as a dedicated Windows box.
"If a native Windows machine runs at 100 out of 100 on speed, our version of virtualization runs at a 90 or 95," he said. "It's very fast and very stable."
I agree. The exception being game playing, since you want 100% of your CPU and virtualized 3D doesn't appear to be fully supported yet.