Music seems to work sometimes with getting started on things. Maybe it does narrow the cognitive space, but this is actually a good thing as long as the mind isn't really occupied with the actual work and is instead full of random, distracting thoughts. So maybe there should be a music player mode which starts normally, but then slowly fades out during the next ten minutes or so?

Music can also alter mood, which could likewise be useful for getting started on working.

This is worth thinking about. I was trying to figure out why people like music during demanding tasks so much, and maybe it's a way of distracting themselves and committing only partially to the task - maybe studying or whatever is too 'painful' to fully concentrate on. (There's a connection to akrasia here, I'm certain.)

One thing I'd be interested in knowing is whether the familiarity of the music matters. I wouldn't be surprised if completely novel music caused a larger cognitive load than familiar music. What about if you've heard the piece of music once, ten times or a hundred times?

Also, what effect does the type of the music have? Lyrics probably make a difference. What about music that doesn't even have many recognizable melodies, such as ambient or trance? What about white noise?

As I said, any music seems to be a negative. I can't speak to ambient or trance or white noise simply because I don't play them (if you're really curious about the full gamut of music I tried out, I can say I tried the majority of my collection, and if you're curious what that entails, see http://www.last.fm/user/gwern ); however, I did try out various binaural beats generated by Sbagen which sounded a lot like white noise and had a similar negative effect.