I know that when iOS 5.0 mentioned 'twitter integration' and BBM like messaging, I had assumed (wrongly) that iOS was going to use twitter DMs in the way that they are using iMessages now. This would be using a person's twitter handle instead of the Apple ID associated with the iPhone. Like I said, this did not come to pass, but it could have been used as a BBM-like messaging service pre-iMessaging.