I was wondering if RIM published an API that allows third party apps on non-RIM devices, like the iPhone, to send and receive Blackberry messages?
BlackBerry Messenger uses the PIN to PIN protocol to transport the messages, so any device would have to be assigned a PIN. Rim does have a program (BlackBerry Connect) that allows some non BlackBerry devices to connect to BlackBerry infrastructure, though it is not clear that this software enables either PIN to PIN or BlackBerry Messenger communications. The iPhone is not on the list of supported hardware.
Also, since the BlackBerry Messenger traffic is routed through the Rim Operations Centre, one would have to have a BlackBerry Data Plan.
I have never seen any information of such a published API, though that doesn't mean it doesn't exist, for the reasons above it is highly unlikely.