Under a new policy, AT&T has started cutting the data speeds of its heaviest users as part of an attempt to manage data usage on its network, reports the Associated Press. Mike Trang likes to use his iPhone 4 as a GPS device, helping him get around in his job. Now and then, his younger cousins get ahold of it and play some YouTube videos and games. But in the past few weeks, there has been none of that, because AT&T put a virtual wheel clamp on his phone. Web pages wouldn’t load, and maps wouldn’t render. Forget about YouTube videos–Trang’s data speeds were reduced to dial-up levels.