I finally completed the MIT Introduction to Biology class which I downloaded from the MIT Open Courseware site. They provide both video and audio downloads of the class. Since I was primarily listening as I commuted to and from work, I used the audio modules. In the version I downloaded (7.012 from Fall 2004), the instructors were very engaging. While I missed a lot of content, not being able to see the visual aids and the drawings on the chalkboard, I still received a great deal of value from the lectures. For example, I can't tell you where the 5 prime carbon is on the base of a DNA molecule, but I do understand DNA transcription to RNA and then its translation into proteins. I would conclude that I captured about a third of the content from the class, and most of the higher level concepts. Thus, it was extremely worth my while.
Having completed the Biology course, I have now started a course on Animal Behavior (9.20 Fall 2005). So far the instructor is not quite as engaging, but it is much easier to follow (less detailed content).