When you buy the 3G iphone, do it in Italy. You’ll be able to buy it unlocked there, I think (a few European countries seem to require an unlocked version). It’ll be significantly more expensive, and I wouldn’t be surprised if it doesn’t work perfectly with AT&T, but it should allow you to plug in a pay as you go SIM.

And yeah, in the mean time, there are Methods of getting the functionality you’re looking for. As you seem to know, in a hypothetical way.