I have installed VMware Fusion 1.1 beta, so see what its like. I have created a virtual machine based on my boot camp partition, and I have installed ubuntu 7.10 via fusion (its just like installing it on a computer, except your 'computer' is a window on your desktop).
I am now compiling firefox trunk on ubuntu, and am installing the pre-reqs still on windows. I am doing these things at the same time, all the while using my base OS X to run my browser and other normal tools.
Yes, thats right, Ubuntu, Windows, and OS X all running on one laptop. all nicely running inside their own 'spaces' window wow.