I had the GoPhone (prepaid) option on my iPhone for the first two weeks. I was "forced" to activate with GoPhone because my credit was denied. NOT because I have bad credit (far far far from it) but because I subscribe to LifeLock (lifelock.com). LifeLock protects you from identity theft by putting fraud alerts on your credit reports. The fraud alerts prevent anyone from getting credit in your name (including yourself) without answering some very private questions IN PERSON. Since iTunes can't ask me those questions in person (face to face), my credit with AT&T was denied.
Anyway, I subsequently converted the GoPhone (prepaid) account to a regular (postpaid) account by going to an AT&T store and applying for the credit account IN PERSON. And I wish it hadn't taken two weeks for AT&T to be able to do this (I tried to do it just 3 days after getting the iPhone but they weren't yet set up for it) because those usage/balance remaing alerts were EXTREMELY annoying! To the point of wanting to just turn the damn iPhone off until I could get it converted to postpaid.
I feel for ya! But it's not just the iPhone that gets those usage alerts. ALL prepaid (GoPhone) accounts on AT&T get alert messages. It's SOP for prepaid accounts and, nobody, not even an AT&T supervisor, could turn them off for me.
Personally, I think it is a torture method intended to get prepaid users to find a way (deposit, whatever) to convert to postpaid and sign a contract!
Mark