Researcher Ashley Deal has determined that podcasts have no inherent value (see Campus Technology for brief summary). I am not certain if she cites my research (I read the summary associated with the link), but I have arrived at the same conclusion. My interest was in using podcasts as a way to enhance lecture attendance and my data indicate that students prefer a written summary. This does not mean that are not useful educational applications.
BTW, the article in Campus Technology links to a very useful white paper (pdf) describing podcasting.
