With latest iTunes update, quite silently, a very nice new feature appeared. Apple started to offer for download lectures from many universities, with wide selection of topics: from environment through politics to mathematics and computer science. Most of the lectures are videos, but you will find also audio tracks - albeit for some topics, audio is quite problematic. Can you imagine doing e.g. differential equations in audio-only ?
The best feature - the lectures are free downloads. See more details here or on iTunes.
I guess now I do have really compelling reason to buy a video iPod :-)