I just installed the WinHEC build of Longhorn in a Virtual PC, and everything is smoooooooooooooth. I actually left the computer, did some laundry, played some Splinter Cell, and came back to see if the thing had crashed remembering the ordeals of the PDC build with a VM. To my surprise, it was up and running. Granted, it was 640x480 w/ 16 colors, but a quick install of the Virtual Machine Addins took care of that. I really is running great on my machine. I should mention that I've been working on my machine tuning it for VMs so it probably runs better than a lot of people out there. Here's what I got:
P4 2.4GHz w/ HT running @ about 3.3GHz, a gig of ram (512 allocated for the virtual PC), and I've got the Virtual Machine running on a seperate controller than the OS (a couple of SATA drivers running in RAID0).
I never did get much into Longhorn because during the last big hype period I didn't have the resources to dedicate a machine, and VM was just too slow for my liking. But so far, so good.