We don't actually know for sure what will happen. The belief among those who should know seems to be that they've been working on re-architecting the infrastructure and overall design of BBM to work effectively for people with multiple devices under a single BBID. This would presumably be because people might have one or more tablets as well as a phone, and they'd want to use BBM seamlessly on any of them without messages going missing, etc.
It's expected they'll have native BBM on the PlayBook at some time, but another issue nobody's really answered is whether WiFi-only PlayBooks will be allowed to play, and if so whether there will be an extra cost. Up to now BBM users have always been paying monthly fees to RIM through a carrier, so whether they'll want to replace that with a direct subscription, or let WiFi-only PlayBook users get it for free, or whatever... anyone's guess.
For the mid term, you've basically got to look at BBM via Bridge *only*, and until we learn more about the BB10 phones I doubt we'll learn more about BBM at all.
Certainly don't expect any third-party API before the fall, in my view.
