I managed to get a working Zimbra 5.0 server on FreeBSD.
It's experimental, and I still have lots of TODOs, but if you're interested you can check how to build and install Zimbra on FreeBSD on my wiki : Zimbra On FreeBSD - Gid0.org
The interesting points :
- light distribution : only Zimbra servlets, libs, jars and config files are installed
- ports : third party software (MySQL, openLDAP, ...) are installed from ports
- no /opt
Feel free to provide feedback.