I recently visited the blog of an instructor at the University I attend (University of Victoria), and in his blog he (awhile back) talked about an instructor in the US who was selling recordings of his lectures to students. I thought this was an interesting story, as given that I'm a TA and occasional sessional instructor at UVic I often use my MP3 player to record my lectures and have always wondered what the reprecussions would be in terms of student attendance and involvement if I made the recordings publicly available.
I know for myself as a student I would still attend the classes, but OTOH I know a *LOT* of students who would fall into that trap of "well, if I can download a recording of that lecture later, why should I go to class?" I do think that there is something to be said for actually attending a lecture over just hearing the audio of it, as things such as body language and the like convey almost as much as what we say. Additionally, you lose the benefit of whatever overheads & the like the lecturer used (unless they are made public as well). Besides, you can't interact with an MP3 recording by asking it questions (or at least not last time I checked). ;)
OTOH, as a student I would very much appreciate the ability to "review" for exams and such by re-listening to entire lectures again. Like podcasts, it would be convenient to be able to listen to lectures as background noise when I'm doing other work, or walking to school, etc. Just seems like it would be an excellent way to reinforce information covered in a class.
It seems to me that there is always a trade-off facing instructors in regards to how much material should be made available to students outside of the class. Too much, and nobody will attend class; too little, and students will artificially struggle with material that perhaps they otherwise wouldn't struggle with.
I sometimes wonder what other instructors think of voice recorders and the like as well. Do instructors have problems with students recording lectures themselves? I personally couldn't care less if a student wants to record what I say in a class (of course, that's probably a dangerous attitude, as sooner or later I may say something in a class which I will later regret saying). I've never seen any students walk into a class and plop a voice recorder on their desk to record a lecture, but I would actually be quite surprised if it never happens (given the proliferation of MP3 players, cell phones & the like with voice recorders built in).