With Apple introducing the new iMessage for iOS 5, there's no odds of having BBM on the App Store. Nada, Zilch.
The Jailbreak community has been passing rumors of getting BBM on to the iPhone interface, but that's aeons ago. Will check out iMessage till then, it's said to release next fall.
Edit: Oops? And I'd like to be a tester too, if possible.
I rennet there is someone or some group that created an app and server to fake the bbm message signature thing and give the iPhone the ability to message a bbm. It is somewhere on mmi, it was in beta the last time I looked at it.