I have the SAME problem! And you are totally correct.
Carrier: AT&T
Device: iPhone 4
iOS: 6.x
I checked my detailed usage from the myatt website, and it says that at 9pm on Saturday, October 6th, I uploaded (sent) 7.9GB worth of data in one transaction while I was at home connected to wi-fi. I have not downloaded or uploaded anything closely resembling even 1GB of data on my phone in months. The only thing that could have been that size was an automatic backup to icloud, which should only be happening if I am plugged in (charging) and connected to wi-fi. I have all other automatic downloads and backups turned off, and I have not downloaded any music or movies.
I have a 2GB/month plan and have not went over that limit in the 13 months I've had my iPhone on AT&T. I was thinking of calling AT&T after I found out I was 6GBs over my limit, and while complaining to some coworkers, was told of the iPhone 5/Verizon debacle. I researched and immediately called AT&T customer service, but did not mention the Verizon/iPhone 5 bug. After almost an hour of explaining that I have everything set to not use cell data service for automatic downloads and backups, and that I was on wi-fi the whole time, I mentioned that I had heard of the Verizon issue. She confirmed that her company had sent an internal memo that AT&T was not affected by the issue, but that they would call me back when my billing cycle closes out, and that they would "take care" of the charges. I am assuming that means erase it, especially since she said no to worry about continuing to use my phone as I normally do, even though I'm already over my limits. I at first told her I would switch cell data off until my next cycle.
I am a network engineer and I work with computers and technology for a living. I know this was some technical issue causing the use of cellular data when specifically set not to. And because of the size of the data transaction, the time, and it being an upload, I know that it was during an automatic device backup to iCloud.
Apple needs to come forth and admit it is a problem with iOS 6, not just iPhone 5, and that it is an issue with other carriers beside Verizon.