Yes, there's an issue and you should call your carrier and get it documented immediately. Does it happen while you are not on the 4G network, then disappear when you are? That's my experience.
Watch your data usage closely. Call your carrier often to make sure your numbers seem reasonable. Complain if they're not.
I noticed the same thing this morning while checking my data usage to make sure I wasn't having the cellular data while on WIFI problem that's been going on.
So I called AT&T technical support and we looked around for about twenty minutes. He confirmed the following:
I was not signed up for tethering and was still on my GF Unlimited Plan
AT&T showed no tethering data usage for my account
My other usage was not insane or weird
That this does not exist in the 4S running iOS6 and seems to be a iPhone5 issue, he checked his own phone to be certain and I showed him where cellular usage was in settings, aha!
He thought it might have been from connecting to a tetherable device, like someone else's open wifi connection from their phone. However, having only had the phone for a week or whatever, and remembering where I've been, I haven't connected to any WIFI that I hadn't previously connected to with my iPhone4. Except in the AT&T store when the guy activated the 5 without my permission (douchecanoe). I don't think the AT&T store WIFI would be beat out by an open tether connection, so that's probably not it and it should ask you to join a network, right? So I don't believe that is the problem.
Also, neither of my laptops or the iPad have tried to connect to a new device at the house so my phone doesn't seem to be open. Not really sure what to think at this point.
I'm glad to see that I'm not the only one with this issue. AT&T is not really aware of the issue, he said I was the first to bring it up to him.