Have you updated your iOS to 5.1? Just confirming as it says "iPhone 4S, iOS 5.0.1" in your thread.
Double check if you have credit in your mobile in order to activate iMessage.
Turn iMessage off and on again, than turn off your phone (long press display off and on button) and slide to turn off. Turn your phone back on, enter your "Apple I.D" in the iMessage window and wait till it verifies everything.
Tell us if it solves the issue.