There seems to be a lot of chat about this lately, just wanted to share that I'm running ZCS 4.0 on a FC4 VM (on a Windows XP Pro host.)
7 Users, uptime about 3 days, and no issues so far!
I allocated 20gb and 512 MB.. The VM is using ~275mb on average and rarely pegs the CPU on my Sempron 3000+.
Obviously these specs wouldn't suffice with tons of users, but I find it encouraging that everything went so smoothly that I'm planning to switch from hardware to VMWARE at work. I love the snapshot feature, I've used it daily, and restored once just for S's & G's with no issues.