Er, what about iChat? Doesn't work with Blackberry Messenger as far as i know, but it works great between iPhone's. AIM doesn't have a realtime app for the Blackberry yet but I'm sure they will soon.
I couldn't imagine carrying both devices around, even the Curve is a bit much for me. I'd rather be shot dead than use a belt clip and the iPhone doesn't fit into my hip pocket very well because it's so long...and awkward with the pretty much mandatory silicon sleeve to keep you from dropping it.