While not being 100% easy sailing, I was able to install ZCS 5.0.5 onto a VM'd Ubuntu 8.04. The guest machine has 1GB RAM and 25GB of LVM/JFS disk space. Really impressed w/ the Zimbra CS. Really like the Zimbra Desktop. I will probably roll the 25 user Network Edition out to my company when Ubuntu 8.04 is "officially" supported. Kudos to the Zimbra Team. Hope you guys don't get swallowed up by Microsoft.
Chazsani