Will it ever be possible to port BlackBerry messenger (BBM) to the iPhone?
It's almost certainly technically possible, but the reason it hasn't been done isn't a technical one, it's a business one.

BBM and the network of users that go with it is one of RIM's strategic assets, and their thinking is likely that opening it up on other platforms would eliminate a reason for people to buy their hardware (which is where they make their money). Whether that's a sound decision or not and whether RIM may change its mind may be up for debate, but at the moment, that's why there's no BBM for iOS, Android or any non-Blackberry platform, not technical challenges.