Partial sidenote: Saw a massive spike in data use which seems to correlate to the install of the podcast app. I don't see a spike when the app is turned off (and the app was set to not use cellular data the whole time).
I think I have a temporary work-around.
Since iOS 5, the behavior of Wifi changed. While it was once persistant; in iOS 5+ it turns off the wifi annenae when asleep. UNLESS you are connected to power. I have checked this with the 'ping' command, and it does appear to be the case with my iPhone 4. I cannot find a non-jailbreak solution to keep my wifi connected save for keeping it plugged in (impractical, but not impossible)
(speculation) the Podcast app is not able to figure out when to STOP downloading material when the wifi is off.
So if you want to sync podcasts- connect to power to stay on wifi. Then be sure to close it when done so it doesn't start up after a call ends.
Amusingly enough, I ended a call with Applecare talking about this very issue when the podcasts started playing. Applecare (tier 1) had no solutions or tech notes about this but AT&T seemed to be aware of this data use spike with iOS 6 (or more likely the buggy podcast app). I sent in a bug report with the data use spikes on it as well.