If anyone uses iTunes for any reason you may want to have a look at something called iTunes U. A number of academic institutions have put up podcasts on various topics. So far I have only checked the listings for the OU and there are video podcasts on there for Astronomy and Planetary Science. They seem to range from 2 to 10 minutes in length and transcripts are available on the site also. About 15 files currently uploaded at present but hopefully this resource will grow over time.