iPhone 3GS won't let iMessage show my number; only email
I've found the best solution to these iMessage and Face Time woes is to just turn the service off, then back on again. That causes it to re-activate with Apple's servers, which should register your phone number as a usable iMessage sender.

The "Waiting for Activation" message suggests that this process wasn't completed properly, and/or there's some issue with Apple's servers. Try toggling it, and if it still doesn't work, try it again later, as it may be a server side issue that you need to wait for Apple to resolve.