Hmm, I used to love this feature too, as if I just wanted to listen, I knew I wasn't burning battery letting my pocket see all the video action.
Hitting sleep shuts down the playback on mine. Boo.
Edit: I just posted on Apple's discussion board, and might leave feedback too asking them to revert the behaviour, or at least give us an option. In the meantime, I found a post where someone has found a workaround that seems to work:
If you start playing the video podcast, and then turn off the screen, and then double-tap the home button to bring up the iPod controls over the unlock screen, and then hit play, you will get audio with no video.