I had that happen to me a few times, notably after making some very large downloads over the 3g network (for the Gameloft game demos).
When the issue cropped up, I also found that my airplane mode wouldn't toggle properly. The phone would continue to operate normally after I tried toggling airplane mode, but it wouldn't actually disable my cell connection, and the toggle buttons (both from the notification bar and from long-pressing the power button) would become unresponsive.
It only resolved after restarting the phone.
Notably, the issue only happened to me after making those Gameloft downloads and playing their games. I was able to make other stupidly large downloads via things like Swarm (a torrent client) without any problem.
I've only experienced the issue while out of town, though, and not at home, since I do most everything over wi-fi, so I haven't exactly put much more effort into figuring out the cause. I uninstalled the demos I had after completing them (Asphalt HD, Assasin's Creed HD, and Nova HD) and haven't had the issue since.