The problem with podcasting is not that you can't put ads in podcasts, it's that podcasts as a medium are inconvenient.

You can't skim read a podcast. You can't easily fill 2 spare minutes by skimming a podcast like you can with a written article.

The same problem exists with online video and talking head "video blogs" FWIW, though it's not quite as bad, as you can still skim-view.

The HTML5 video/audio elements will enable you to increase the playback rate, which will somewhat alleviate this problem.