My iMessage was working fine up until yesterday when it stopped working completely. The phone I am supposed to be iMssaging still has iMessage yet my phone isn't even giving the option of sending as an iMssage now to this person, just a normal text message.
I had same problem... Facetime wouldn't work either. Facetime gave me a clue - said the time settings were incorrect. I just flew into LAX from Philadelphia - when I adjusted my clock to New York time (manually.)
I turned automatic time adjust on and it activated iMessage using the iPhone settings - General settings.