it takes awhile to activate... I've had this issue since Beta 1 until now. You just have to toggle it on and leave it. I have tried turning phone on and off, doing it with wifi on or off.. Nothing really makes a difference.
What I did notice, is that I could iMessage after a short time of waiting for it to activate, even WHILE it still said "waiting for activation." So there might be a slow communication between the phone and servers. Try testing iMessage with someone who you know has iMessage. Probably hard to do at the moment since it's just rolling out now. Also, when you're typing in someones apple id or phone number for iMessage, don't just type the message and send. wait for the send button to turn from green to blue, then send it (as long as the other persons iMessage is active, this will always happen). I did this a couple times to kind of kick the phone out of the "waiting for activation" time frame.