i'm having this same problem, but i don't know if it's really a problem. even though it says it's waiting for activation, i'm able to send and receive imessages just fine. i confirmed by specifying my friend's imessage address, my outgoing messages are blue tinted and say "delivered", and he replied.
did you register your phone number?
i read somewhere on the internet you need to verify your phone but what if you're hacktivated?
in the message settings, under caller id i can only choose between email addresses.
I didn't register anything, and I'm pretty sure you just link your email addresses to it. It's working as far as I can tell but still says waiting for activation. My iPhone 3GS is jailbroken and unlocked on tmobile, using the iPad baseband.