There have been various ways of getting iPhones onto AT&T's prepaid GoPhone plans pretty much since the beginning, but for some inexplicable reason (hint: contracts are lucrative), the carrier is closing the door on prepaid in concert with the release of OS 3.0 this week. To be clear, we have absolutely no idea how AT&T would reliably detect whether you're running 3.0 on your iPhone -- unless Apple were to report it to AT&T using data culled from iTunes, and we somehow doubt it's happening that way -- so in all likelihood, they're just assuming most folks will move to 3.0 within the next few weeks and are using the release as a convenient line in the sand. New iPhone 3G and 3G S owners will have a hard time signing up for GoPhone altogether, and legacy customers are being told that the upgrade "may impact the data service" unless they move to postpaid. Actually, it's even more stratified than that: only original iPhones are being allowed to stay on Pick Your Plan with the unlimited data add-on, while 3Gs have to move. To be fair, this has been the policy all along -- 2G on Pick Your Plan, 3G not eligible -- and it seems they're just now deciding to lay down the law and bring everyone into compliance, but that doesn't mean we have to like it.