The problem I have with iMessage is that i end up having to start a new thread with the exact same contact email or phone number just to make the message send...and also, sometimes if i send the message from my iPad, the messages dont come through on the iPhone until I start a new iMessage thread, or even worse the next day!
Its almost as if there is some kind of block on the system that doesnt unblock itself until you start a new message...
Sort it out Apple....your taking the piss