I had the same experience with ATT that Michael had. They reversed the charges without giving me any grief at all.
In my case, I noticed the problem pretty quickly as I was traveling and on a pretty restrictive european data plan. I'm on an unlimited plan in the US so I had never paid too much attention to the amount of data of use. I did look before traveling and my average use is about 600MB a month and after iOS 6 it jumped to over 4GB.
Once I noticed what was happening I deleted my iCloud account and the podcast app which stopped the data leakage. I have since upgraded to 6.0.1, re-enabled iCloud and switched to "Downcast" for podcasts as I never liked the Podcast app interface. With all of that being said I did notice a 30MB upload last night while I was sleeping so something wonky is still happening.
At the end of the day ATT gets a 10 out of 10 for customer satisfaction and Apple gets a 0.
In the event that someone from Apple is monitoring this thread (Id be surprised if there wasn't) I'd like to say this:
We pay a premium for Apple products because they are easy to use, reliable and Apple is well known for their customer service. There is\was obviously a bug in either the iCloud or Podcast app that caused the iPhone to use excessive amounts of wireless data when connected to WiFi. The right solution here was not to ignore the problem and let your valued customers twist in the wind. Some open communication and reassurance would have gone a long way with us and would certainly have saved us some grief and agrivation. I am the owner of 3 iPhones, an iPad and a MacBook Pro and have been a very outspoken champion of Apple products for many years. However, this situation was handled very poorly and it's certainly enough to give me pause before making my next purchase.
Regards,
Al-