Upgrading my iPhone 4S to iOS 7 worked - but now iMessage doesn't.

I recently updated using the tutorial to iOS 7 on my iPhone 4S. I followed the tutorial's directions - everything worked smoothly, but now my iMessage doesn't work. iMessage says it's enabled. In terms of the settings, everything looks fine. It doesn't give me any errors/waiting for - but when I try and send an iMessage, it can't send it. I understand that this is not the official release, so bugs are going to happen. I'm just curious that some 4S users have reported this problem - and some don't. Thoughts? Thanks.