I don't much of what is said about how the messenger works on a phone is correct.
Someone said the network can tell when you start to compose an SMS message. I say they can't using regular SMS and if someone thinks they can, they better be able to prove it by citing a credible source. SMS uses the old GSM protocols for cellular broadcast systems. It simply doesn't send anything until you press the send button.
Now a phone manufacturer can put additional software into the phone so that it can detect log this and send it (e.g. via the internet connection) but if they did this it would be similar to the CarrierIQ privacy scandal and it would be all over the tech news (again).
Now IM (instant messaging) is of course a different kettle of fish and you'd expect the IM service to know when you start typing. However again there's no reason the phone provider would know unless they were one and the same, and in many cases they aren't.
In the case of blackberry messenger to blackberry messenger, sure blackberry have their software on the phone and on the server, they would be able to know IF they had appropriate logging systems on their servers (which they almost certainly do) but that's not an SMS message.