The killer app in messaging will continue to be presence awareness. I have yet to find an app that effectively communicates that I'm away from my desk after 5 minutes but still available through my mobile device.
We've been debating whether to differentiate between "online" and "mobile". The obvious answer is to do so, but it might be that with phones being better etc some people prefer to just use the mobile app for much of the day, especially if they only have one monitor...