Fusion will let you run Windows and Linux 32 and 64bit guest operating systems on top of OSX without needing to reboot ala bootcamp.
One nice thing about using Fusion over Parallels is that with Fusion you can easily install and run the hundreds of pre-made Virtual Machines in the VMware application library. There may be other advantages for Fusion, but I've not looked into it ... mostly cause I don't have a Mac. I'm sure comparisons will start popping up all over the web once mac users start playing.
Now we just need a way to run a virtual OSX guest ... I mean we can do everything else now ... run guest Windows and Linux on a Windows host, Linux host, or OSX host box. Since OSX is based off FreeBSD, I'd think making it run in a virtual environment would be easier than Windows??
I think many of us Windows IT dudes would more likely play with OSX if we had an OSX VM on our windows boxes ... not the other way around thank you. Why? (1) Cause I don't want to buy Mac hardware and (2) 'cause I need Windows as my main OS. I'm sure there's some project out there working on this ... maybe???
Next question is will Fusion be a free virtualization product like VMware Server? VMware "forced" Microsoft to make it's virtualization product free ... it will be interesting to what happens with Fusion and Parallels.