Last summer, Geotrust added a new intermediate CA to their chain of authority to allow for 2048bit keys. This CA would not have been included in server configurations that came out prior to July 2010.
The result is that some older servers which haven’t had that CA added experience trust issues for browsers trying to make a secure connection.
We’ve put together a quick help article at http://help.easydns.com/article/geotrust-ssl-certs that explains the situation, but all the details you really need (including the steps to install the CA) are at https://knowledge.geotrust.com/support/knowledge-base/index?page=content&id=AR1422
If you’re using Geotrust/Equifax certs, please make sure that you have the CA added to avoid these issues.