This is by far one of the most asked about topic at the moment. Have you spent any time searching for this on either the community or in general on internet with a search engine? It is related to changes that Apple have forced on any 3:rd party vendor with their change for something named Apple Push Notification Service, or APNS in short. If you Google that you'll find plenty of material on what you need to do. In short you need to be on specific versions of CM and have Push Notifications turned on.
