Here is the situation with my AT&T iPhone 5, iOS 6.0.1.
1. Settings-->General-->Cellular-->ON
2. Settings --> Wi-Fi --> ON and connected to good network and verified connected to the Internet
3. Settings-->Podcasts-->Use Cellular Data-->Off
Then, download a podcast --> No cellular data used
Next, interrupt Wi-Fi by either turning Settings-->Wi-Fi-->OFF or unplugging your Wi-Fi router
Then, download a podcast-->Cellular data is used for entire download until Wi-Fi is reconnected, even though Settings-->Podcasts-->Use cellular data is OFF. This occurs without warning unless the single podcast is over 50MB, in which case you get the "Connect to a Wi-Fi network" but it still continues the download over cellular.
I suggest that Settings-->Podcasts-->Use cellular data-->OFF should mean Podcasts should never use cellular data, regardless of the status of the Wi-Fi connection.
I too have had huge AT&T data bills because Wi-Fi either dropped out or wasn't sufficient during a podcast download, even though Settings-->Podcasts-->Use cellular data-->OFF.
Am I missing something here? This appears to be a bug in iOS 6.0.1. Until then, to avoid using cellular data when downloading a podcast on Wi-Fi, I suggest turning Settings-->General-->Cellular-->OFF.
Jim