If you generate enough SMS messages through an official SMS portal, they begin to start paying YOU by the message, because they are making so much in SMS charges.
What's the source of your statement? I've talked to few cell companies and they never offered anything even remotely as attractive. Best we could do is get a discount. You're talking about revenue sharing. I've never seen anyone extract that out of them.
TV shows do it all the time. You don't think they encourage people to vote multiple times just for the hell of it, do you?
Interesting, but does it still work if you are sending far more messages than you are receiving? I haven't used Twitter, but I imagine that for every SMS they receive they have to send out dozens of update SMS (to all "friends").
How would they make money in the long run? Sell pro tools and statistics... similar to the way Feedburner makes money. Just study Feedburner and you'll see plenty of parallels.