Today, I renewed my RapidSSL certificate, and it took me the better part of the day to figure out how to get it installed.
As of december 9th 2010, RapidSSL needs another ca_bundle.crt than what was stated in the Wiki.
I edited the Wiki page: Installing a RapidSSL Commercial Certificate - Zimbra :: Wiki
This message is just to keep you from pulling your hair out.
A check wether the Wiki is correct now is welcome - I fiddled a lot with files and certificates before I got it working.