A friend of mine has iPhone 4S with iOS 5.0. I can iMessage him using his apple id fine. However, when I try to use his cellphone number, I never get the light blue send button. It stays green all the time. This seems very strange to me. Any explanations?
iMessage sends a text message to get activated on the first boot of the phone. Maybe this text message didn't arrived at Apple, so that his iMessage is only activated for use with his Apple ID.
He could try to deactivate and reactive iMessage.