UPDATE: Official Zimbra VMWare build is now available at: http://www.zimbra.com/vmware/
We've now got a LiveCD (500 MB) and a VMWare Appliance (1.5 GB) up on the download page. These aren't recommended for production use and are of demo quality only. Basically enough to download, run and have a portable demo or playground.
The LiveCD one is super cool IMHO. It come's pre-loaded with some sample messages, Firefox 1.5, Thunderbird with preconfigured IMAP accounts, and a quick readme. Lets you send mail back and forth to the local test users or out to the internet if you've got a connected machine.
Thanks to Marc for putting these together! Please post your feedback or comments. If you find a problem please file a bug in bugzilla.