I put some thought into this as well, there are some decent and fairly inconspicuous omni directional microphones that you can buy and plug into your laptop. I figured I would wait and see what the classroom environment/acoustics were like before making the investment though. Also, if you go the laptop route, make sure you have appropriate software to compress wav's to MP3's if you don't want to fill your HD fast.
On a side note, I think that recordings are generally only appropriate for the day that class happened. If you're going over your notes and come to something you remember discussing but can't put your finger on it, bam, load it up, cue to that section of class and you're in. As soon as you have a couple days saved up, it becomes inefficient to go back and listen in.