BBM surely puts a dent in SMS usage on Blackberries, but not as much as it could. That's because it puts the burden on users to figure out that the person they want to chat with also has a Blackberry and to get their PIN from them. Unless they know both of these things, they will use SMS.

iMessage, on the other hand, replaces SMS. It's the one and only way to send text messages on the iPhone and it will avoid SMS whenever possible. Users won't even think about what underlying protocol they are using. But they may notice that texting between iPhones has nifty new features and doesn't show up on their phone bill. Apple is trying to take users out of the game. If that happens, it becomes just a technical battle that SMS can't win.

However, SMS won't completely die until all the phones are using the same federated IM protocol. We're not going to have The One True Phone any time soon so I wish they would all just swallow their pride and standardize on XMPP or whatever. If just Google and Apple did this, they could probably kill off both SMS and Blackberry. Seems to me that would be worth cooperating on.