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 Inc. 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.