Cross Platform messaging apps aren't exactly new.. Live Profile is just one of many. Anyone who has BBM has to be first convinced to step off of BBM and thus be convinced to use Live Profile. The advantage here actually plays in RIMs favor depending on how you look at it.
Those who left BB can now talk to those still on BB..and those who leave BB for another platform can still talk to those on BB.. The key point there is that someone, somewhere along that line is using BB and if that's the case then RIM already has collected the money.
Not to mention, text messaging has been around forever now and no one has jumped ship over that..and that is the first, true cross platform messaging service. Try it, lol.. I promise it works on BB, iPhone, Android and heck.. Even dumb phones. :)