I would also think that testing and certifying CentOS would require a minimal amount of effort on Zimbra's part, with it being so very close to RHEL.
Centos provides RHEL's quality without the huge price, especially if you're doing clustering.
Ubuntu really isn't ready for the enterprise market, let alone the enterprise server market. Don't get me wrong, I love Ubuntu on my desktop at home, but that's where it belongs. Lack of Commvault support alone is a deal breaker for us.
Definitely voting for the bug!