Blackberry has locked down BBM pretty well. The only way I think they would ever have an attempt at doing this is if they knew how/where BBM's get transmitted from. If they somehow were able to intercept the bbm on the network, and reroute it to another location, they could create a web based messaging service to do it. I dont think they have those details...I dont think anyone does, besides the guy who created the source and a few select bb employees I bet.
Its the only thing REALLY keeping them a float above other smartphone competition.