The Wall Street Journal has an article up today saying the iPhone uses more data than your average smartphone and suggesting:
In the short term, carriers should abandon unlimited data pricing plans. Both AT&T and Verizon Wireless already charge extra for heavy users with wirelessly connected laptops. They will have to contemplate similar strategies for smartphone users.
Short gloss -- yes, the iPhone will strain AT&T's network and the profit on the iPhone plan is probably a bit less on a per-user basis than it is on, say, a WinMo phone. However, ARPU is "average revenue per user", and the iPhone makes AT&T's ARPU look high and that's still what people pay attention to. (And even with reduced profit, AT&T still makes WAY more ARPU and profit per-user with the iPhone than they do off a feature phone.)
As for abandoning unlimited data: like the cable companies, every provider wants to but nobody has figured out a way to do it that doesn't make end users livid. So, we doubt that AT&T will pull that particular trigger.
Finally, network upgrades aren't just about giving users speed and better experiences with fewer dropped calls and the like -- they're also about the network lowering their per user cost, and that's certainly in AT&T's best interests as well.
So you're right, WSJ, that the iPhone cuts into some of AT&T's profit levels, but not so much that it's going to be a problem. There's a reason they're so desperate to re-up their contract with Apple.