SSL certificates are a constant source of pain for Exchange administrators. With Exchange 2007 and 2010 so heavily dependant on web services, getting SSL setup correctly is important for correct operation.
A lot of SSL certificate deployment is now being done for mobile device support, and then you open a new issue - SSL certificate compatibility.
Recently I found a large list of SSL certificate and client compatibility.
It is from a Danish SSL reseller called FairSSL:
Most useful for mobile platform compatibility, the combinations it lists are significant.
On the same site they also have a tool to verify that your SSL certificate is installed correctly. Most of the SSL vendors also provide this, but if you don't have the login details (perhaps because the certificate was just supplied to you) then it is a useful service to have:
With more SSL providers now using intermediate certificates to issue the certificates, rather than the root, getting the certificates installed correctly can mean the difference between SSL working and not.