Update: An AT&T spokesperson has clarified this situation for Macworld. Only the very small number of original iPhone users signed up for Pick Your Plan without an unlimited data plan are affected here, and they're simply being asked to add that unlimited data plan to their current account. iPhone 3G (and 3G S) owners are not and have never been eligible for Pick Your Plan; Pay As You Go accounts were never available for any iPhone customers.

Chalk up another win for AT&T’s in the competition to become the world’s Most Unpopular Provider. The wireless company announced yesterday, by text message to subscribers and by statement that most of its prepaid customers would no longer be eligible for iPhone service come June 17’s iPhone 3.0 update.

Pay As You Go rate plans are not approved for AT&T iPhone customers. Customers currently using an iPhone with a Pay As You Go rate plan should call 800-901-9878 so they can move to an approved rate plan with the required unlimited Data Plan for iPhone.

AT&T currently offers two types of prepaid plans: GoPhone, its “pay as you go” plan, and Pick Your Plan, its “prepay once a month” plan. AT&T’s statement says that GoPhone will not be available for either original iPhones or iPhone 3Gs; Pick Your Plan will only continue to work for existing subscribers using the original iPhone, as long as they have an unlimited data plan. Current Pick Your Plan users who don’t have an unlimited data plan will be asked to add one. iPhone 3G users are not eligible for Pick Your Plan.

According to Erica Sadun at TUAW, who’s been investigating this issue, all pay-as-you-go users are being strongly encouraged to sign up for a postpaid plan, which includes making a new two-year commitment.

This comes across as kind of money-grubbing on the part of AT&T: since original iPhone users paid an unsubsidized price for their handset, it seems as though they should be able to use whichever plan they want—they’re still forking over money, after all, just perhaps not as much as AT&T would like.

For 3G (and, presumably, 3G S) owners, the situation is trickier, given the subsidy that AT&T absorbed. I’d imagine that AT&T is banking on the fact that its iPhone customers are sufficiently addicted to their device by now that they’ll switch over to the contract plans just to be able to keep using it and, for better or worse, it's probably true.