I'm an undergrad (computational physics), so I imagine my "working" environment is a little different than many of you. That said, I own a nice pair of over-the-ear headphones and frequently listen to music while working. When I'm learning something brand new, or reading, music does tend to be a distraction, and I avoid it. Most of my study time, however, is spent working out practice/homework problems, writing code, or just playing around with equations to get a better intuitive feel for them. When I'm doing things of that nature, I find music really helps me get into a groove and concentrate for longer periods of time without getting distracted.
In short, for me: "doing something actively" = music, "learning/reading/absorbing" = no music.
I think I probably have a somewhat unique take on why the music helps me: at least part of it is that I have a bit of a psychomotor agitation issue (probably, I believe, due to mild OCD), where I often feel a need to shake my leg(s), bite my pen, etc. It's not much of a problem, in that can stop doing these things easily if I think about it, but when I'm concentrating on something else, I'll find myself unconsciously bouncing my knees, swiveling my chair back and forth, or even picking at my thumbs, etc. -- I find that listening to "chill," but still sort of upbeat music helps me feel more comfortable: I naturally bob/move my head to the music a little bit, and I actually end up having significantly less "agitated" movement, plus it all feels more natural when it's to the rhythm of a good song. This allows me to focus on what I'm writing/doing without being distracted by my own unconscious movement, and I get into a zone where I am much more productive than I would be otherwise.
Some examples of artists I listen to:
On the more relaxed end: Nujabes, Emancipator, Thievery Corporation, Youth Lagoon
Nujabes - Aruarian Dance:
http://www.youtube.com/watch?v=TUwRGPxCG_Y
A nice 30min instrumental mix:
http://www.youtube.com/watch?v=avUOUiUsAzE&feature=related
More upbeat: Reptar, Phantogram, Vacationer, Pretty Lights, Ratatat, Surfer Blood, Alt-J, Cut Copy
Sometimes I throw some Reggae and oldies in there too.