The guys and gals at Zimbra have been hard at work. Check out the new release of Zimbra 6.0 Release Candidate 1 (release notes). One of the standouts for me is, it now is fully compatible with Fedora 11. I upgraded my mail server running Centos 5 this weekend, no problems. I am contemplating going to Fedora 11 to be able to run the 64 bit release of Zimbra.