Bottlr: Chatroulette for SMS iPhone App
itunes.apple.com
itunes.apple.com
It's a great idea - my little sister sends a ridiculous number of texts a month (as do all of her friends). As far as they're concerned email, facebook and twitter are all for "old" people.
She's well informed. Technically, Twitter and FaceBook are for over-18s only and e-mail hasn't proven portable enough to appeal to most youngsters. In ChatRoulette's case, that's certainly a good thing, with perverts only a click or two away in most cases :-)