This subject has arisen more than once on this board, and I thought that a stuck caliper was bound to be the cause. (The squealing was only when the car was rolling and the brakes were not applied.) Also I had found that lubricating the caliper slides fixed the problem -- for a while.
Recently, however, I bought new wheels with my new snow tires and mounted them myself, being careful with the torque wrench on the lugs. To my surprise, the squeal has gone away.
I think somebody posted a short note the last time the subject was raised, saying "I'll bet it's the wheels." Turns out, he was right -- at least in my case. I guess the compressed air wrench the previous guy used in mounting my tires warped the disk.
Thought you'd like to know.