Podcasts just got a bit more than linear non-scannable low-usability sound files they used to be. "Enhanced podcasts are AAC encoded audio files (m4a or m4b, m4p) with new data inside them that can be time based, like an image or a URL to appear at a certain time." Make Magazine knows everything about how to make one (via Random Culture).
And AdFreak wonders about the maximum length of a podcast ad. 15 secs, no more.