We're happy to announce that we've released Zimbra 4.5.6 for Red Hat Enterprise 5, and that RHEL will now be an officially supported platform in the 4.5 release cycle.
Open Source users can grab it from:
Network Edition Customers can grab it from:
We've created a migration wiki article to help in your migration:
Upgrading Zimbra from RHEL4 to RHEL5 - ZimbraWiki
We know that several Open Source users are running a modified version of ZCS on RHEL 5. You should be able to simply run the installer, and the system should upgrade you to an official version.
The Zimbra Team