one of the many advantages - stability for large deployments (solaris still kicks linux a$$ under very large load/cpus/memory), zfs (still light years ahead of anything from linux/veritas), zones (possibilities of this with zimbra are very exciting - they are so lightweight compared to normal VMs), dtrace, etc etc etc. solaris is still years ahead of linux as an enterprise OS.
i spent many many hours in conjunction with sun microsystems creating a pretty stable, bugfree, cleanly implemented patch to the 5.x tree, and we also provided binaries for both server and desktop. it was quite well tested and run by several community members here. the patches were submitted to zimbra but they showed zero interest in taking them - not even a 'your patch sucks clean it up and we'll think about it'. for that and other reasons we gave up. shame as the patch abstracted a lot of the linux specifics to the source environment - i also hacked together a 99% working FreeBSD patch in an evening from the solaris patch.
don't bother dreaming about zimbra under solaris or any other OS other than the supported OSs. strongly recommend you just suck it up and install on a RHEL box somewhere. sorry to tell you that.
if solaris rises like the phoenix from the ashes of the oracle buyout (don't hold your breath) and becomes a force once more, maybe then it's worth looking at again.
rip solaris, you rocked the joint one upon a time...