Just a FYI to anyone reading this thread...
I had several 'strange' events happen when using my 4S. I activated my 4S and restored my iP4 backup to it using iTunes. The iMessage (and FaceTime) "Waiting for activation..." was just one of the 'strange' events.
After trying everything found on the internet yesterday without success, I finally decided to bite the bullet and start fresh. I executed Settings->General->Reset->Erase All Content and Settings. When that process completed I reinstalled all of my apps, music, etc., which was a major pain, I'll admit but for me it was worth it. Since then I have not had a single 'strange' problem--or even a normal problem. Everything has worked flawlessly.
I am convinced that there is something about restoring from a iPhone 4 backup that caused my problems. It may be unique to my situation, but with so many experiencing similar problems I wonder...