Well, I think I'm going to take a stab at Ubuntu 8.04 server with Zimbra 5. The videos they show make it look pretty impressive and there is a detailed walkthrough on how to get it up and running out on the net. Wish me luck!
What would be really nice is a LiveCD that stores all data/messages/settings on a hard drive but essentially is a full proof load and forget solution. That would be nice...