I was just talking to casey using the GMail version of GTalk and can see whether he's logged in via his phone, via GMail or via some other device.
Not sure about BB messenger, but GTalk can save your chats (just like SMS) so if there was something important that you needed to refer to at a later stage, you can search through messages using terms. very useful.
also, there's an app called cnected - (i think that's how its spelled); one user described it as the closest thing to combining android with bbm, but still needs a lot of work.
i think its just an app that is available on both OS's. it would be good if meebo could add something in. but then if that was left logged in, in the background and everyone did that, that would be the same as using BB or GTalk, just via MSN, facebook, yahoo or whatever.