Only a matter of time until someone figured out how to make money off NOT charging for access to a free capability. Apple will make millions just off of “it’s free on iOS and your friends are already using it” hardware sales.
That’s been the problem: ubiquity. There’s enough iOS users to make transitioning from SMS easy; few others can provide that “everybody is doing it” base.
It isn’t ubiquity as much as ubiquity tied with simplicity. There are several (free) IM apps that work across platforms already. Many designed to mimic BBM as well. They all require some type of registration process and so forth. It is more drawn out, and not worth the trouble when you can just use SMS.
I can’t figure out why Google never released a native GTalk app for iPhone. That would have gone a long way toward accomplishing this in the US at least.
Still have a long way to go on ubiquity as well. I’m down to a precious few without a smartphone, but as it happens the most frequently texted is not on a smartphone yet. So that isn’t happening. I would also say whatever does away with SMS has to be cross platform because competition is here to stay.
Smartphones have, pretty much from the beginning, had access to AIM, Google chat, Yahoo chat, etc. I had an AIM client on my Treo. The main advantage of SMS, as you point out, is ubiquity; the simplest "feature phone," as the kids are calling them these days, can do it.
I don't see iMessage as a direct challenge to SMS. I see it more as a feature-for-feature challenge to Blackberry Messenger, which is cited by a number of Blackberry users as a reason to stick with that platform. With receipts and encryption, iMessage eliminates one reason for individuals, and especially companies, to stick with Blackberry.