Sun/Solaris - back with a vengeance
Yes, Xao - you're 100% right.
Sun now has the best value hardware offering by a long shot - apart from competitive pricing and support plans, its AMD and SPARC servers are winning world records almost daily - and Solaris 10 has compelling features over the other possibilities. Anyone who believes otherwise hasn't done their research.
In fact ZFS was the killer feature for us - it has integrity guarantees nothing else can make. But there's also lightweight virtualisation, and the list goes on. We bought a couple of X2100s but the X2200 M2 is even nicer.
With respect to Zimbra, it's not an easy port, I've spent a few days working on a port to Solaris10/x86 and only got third party done (stalled at Ant for Zimbra itself). I hope I get a chance to work on that some more. The Zimbra build process needs some general polish, description and organisation however, it is a bit rough around the edges, or was, around revision 70 when I jumped in.