The days I forget my iPod are horrible, unproductive days.

I think that has to do with my noisyish work environment more than anything, since I can code at home with or without music. Though it may also be related to the way working at work works (sit at your desk and produce stuff for the next 9 hours)...

But as far as music goes, I'll listen to anything I like and know. Anything new is distracting until I've heard it a few times, but that's about it.