I was on the phone with both Apple and AT&T for over 3 hours last night. Both Apple and AT&T claimed to have had NO reports of cellular data being used, instead of wifi, and no reports of excessive data usage by other AT&T iPhone users.

There's no doubt an issue here as I haven't used more than 37mb in a billing cycle prior to September 2012. Suddenly, my September billing cycle showed I had used 130mb! My new billing cycle started October 12th and by 6:04 pm that night, I had used 56.4 mb. Please note I had been connected to wifi that entire time as I had been working from home. 

Apple had me do a restore, which didn't work. Most everything is off such as Locations, iCloud, etc. Email is set to be fetched manually. The only items in Notifications are phone calls, messages, and my calendar. 

Having said all of that, earlier today I read on the Apple board of a user who turned LTE off but left Cellular Data on, while at home connected to wifi. That individual noticed that cellular data was no longer leaking through wifi and no cellular data had been used.

I decided to try that as well, about four hours ago, and have registered no cellular data usage through DataMan or through AT&T (*3282# from phone) either. And I was connected to wifi that entire time.

I have no explanation as to why that's working for me, as it truly makes no sense, but at least cellular data isn't going through wifi. Again, I have Cellular Data on but LTE off, while on wifi. 

Thought maybe others might want to try it and hopefully get the same results as I have. Thank you for reading my post and I truly hope Apple comes through with a fix shortly.