I find your recent thread stating centos is not nor will be supported comical given I had a 3 way voice conference with Zimbra execs in September that said the opposite when I was renewing our NE contract.
I have used centos 4.5 with zimbra for 2 years odd now with no problems, only issues I've had have been zimbra ones. I was told at the presales stage that centos tickets would be supported and had this reconfirmed in september and have this in writing.
That said I have had no problem moving from 4.5.x to 5.0.0 on centos 4.5 64bit using the RHEL4 64bit packages and I doubt I will move to another platform in the future. Certainly not going to deploy ubuntu thats for sure, maybe migrate to solaris when / if it gets supported.
Have these issues you have reported been with centos 5?