It would appear as if the new root CA from Geotrust isn’t natively trusted on Android devices – which means you can get cert errors even if your client facing cert seems 100% ok (this also appears to effect Windows Mobile 6.5) – basically there’s a missing link in the chain- this link in the forums goes through the implications and the fix.
http://code.google.com/p/android/issues/detail?id=10985
Basically, you’ll be looking to install the intermediary cert on your server:
https://knowledge.geotrust.com/support/knowledge-base/index?page=content&actp=CROSSLINK&id=SO15687
which isn’t too much hassle for compatibility with your mobile devices again!
Next - Next - Next - Finished 