Joy Davia at the Rochester Democrat and Chronicle has written an article providing etiquette tips for using the iPod in an office work setting. Here's what I got from the article (restated in my own terms):

1. Consider how co-workers or your boss will perceive your iPod use. Will they think you don't want to talk to them? If so, are they correct? And is that OK?

2. When a co-worker or boss approaches, do you remove both earbuds? It is disrespectful not to.

3. Do you bother co-workers by singing, head-bopping, or tapping? (Tapping is FreeiPodGuy's failing.)

4. Will it appear that you are not doing your job?

I'll one of my own: does the iPod actually keep you from doing your job? I've found that listening to music helps me concentrate, but listening to more information-based audio (like tech podcasts) makes it difficult to concentrate on work that requires deep thought. When doing mindlessly repetitive tasks, podcasts are great. When doing technical writing, music is better.