Sooner or later (you can probably set a stopwatch going), Apple is going to be hit with a massive antitrust suit, by playing the ringfencing game.
The limit I'd expect that is acceptable in law is Mac OS X's locking to Apple hardware - beyond that, the development of a platform and allowing 3rd party applications means by it's very nature that Apple can't and shouldn't be able to control what goes on it.
At the end of the day, with the iPhone OS 3.0 supporting age restrictions and parental controls (something Apple needs to beef up a bit on the Apple ID, ie having a more stringent age verification, possibly using credit card data to do so), so that underage users cannot get access to adult-rated software. Then all Apple needs to do is simply force all apps to be rated by the ESRB in the US and institutions like the BBFC here, in precisely the same vein as games go through.
That is all Apple needs to do, and if it resists, it should be forced to.