I kinda doubt that's been Apple's plan because if that was their plan, it's a terrible one. It's not an obvious feature to people and they've never done much promotion of it. I don't think anyone has ever said "well I guess I'll get the iPhone because it has iMessage." it just isn't that big a feature and hasn't been promoted at all.That's always the case. Apple has always done that in an effort to get people to get iPhone's because their friends and family have them and it will be great... Or my wife has an ipod so I keep my iPhone so we can chat...
Who knows, we'll see what Apple does with it now that all the litigation is done... for the moment.