If you use the native iOS client for Sametime (which is great by the way) then you might want to make sure you Sametime Administrator is aware of this:
Abstract
Before October 2012, Sametime administrators must download and apply the latest security certificate for iOS traffic. If Push Notifications have stopped working for the native iOS client, this could be due to the SSL certificate for connecting to the APNS server having expired.
http://www-01.ibm.com/support/docview.wss?uid=swg21605219&myns=swglotus&mynp=OCSSKTXQ&mync=E