My own children aren't old enough for homework yet, but I am a teacher, so I will weigh in with a teacher perspective.
When we are finished with our lessons in class, if students have work time (not a quiz, essay, or exam), I do allow them to listen to their I-Pods. These students have grown up in a world much different than how I grew up. They are "plugged in" all of the time. Many do concentrate better if they have background noise, and grades are typically higher on the activities where students are allowed to listen to their music. I questioned it at first, but then I realized that when I do my assignments for grad school, I turn on I Tunes first. When I was doing my undergrad, Dave Matthews was playing through almost every essay I ever wrote. When I write quizzes, tests, or lesson plans now, grade papers (except essays), even when I am reading my textbook, there is usually noise in the background of some sort, music, TV, or the kids playing.
My hubby and I have talked about it for our kids, and both are very musically inclined. They love to dance and sing, and we will probably allow them to listen to music while they work, if they want to, and provided that there isn't a drop in their performance in school.