The clever thing about iMessage is that no-one even knows they are using it. On iOS at least, that's going to make it very hard to beat.

On Android there is more scope, but if, as expected, Google launch their cross-platform messaging service at IO, it'll be in direct competition with BBM. It'll be interesting to see if either brings a killer feature- I have a horrible suspicion that Blackberry will deliberately hobble their featureset on Android and iOS in order to push people towards Blackberry devices. That would be a horrible, horrible mistake.