Alright, here's a data point that might throw a monkeywrench into this situation...
I have an ipod, still on 5.1.1, that I drive around all day connected to a verizon wifi hotspot thingy with a 3GB data plan. Usually, I download the couple of podcasts I listen to at home before work, but if I forget, I'll go ahead and download it out on the road, they run about 40 MB or so. The only other thing I do is a few emails, a few google searches, and the occasional look at facebook.
In the past, I would sometimes get the 50% usage email and chuckle because the cycle ended the next day.
Yesterday I got the warning that I've already used 1.5 GB since the 10th. Going onto verizon's site, it looks like I'm using 3-5 times more data than I would expect at approximately the times I would be listening to the podcasts.
Maybe unrelated? But I've literally never used data that fast in the 2+ years I've been doing this. Even when I first got on Pandora and really listened to it a lot, I didn't start getting data warnings until I was 2.5-3 weeks into the cycle..
So, TL;DR This might be a problem with the podcasts app or some other phenomenon unrelated to ios 6 per se.
I'm just putting it into and out of airplane mode for now, kind of a PITA though.