The only problem is that most people will stick to the unlimited SMS plan. If they have a small data plan (500 MB for example), they'd be using their limited data.
I feel I should explain myself. I understand iMessage is much more than SMS and I love it so far. What I am saying is that is has been difficult to convince friends to use it. They still require a SMS plan since not everybody has an iPhone. Using iMessage will cut into their limited data, while using their SMS plan will be unlimited. Once more people have iPhones, we can ditch the SMS plan and use the "cheaper" data.
I'm curious what the typical texting plan and actual usage numbers are. I have the minimum (400, I think) and I never go over it, but I'm also past the whole social butterfly period in my life.