I've heard that some XTR brakes have a problem where one piston gets stuck in the caliper. This means that when you squeeze the brakes, only the other (unstuck) piston is free to move, so it moves too far and when the brakes are released, does not go back to its origional position, resulting in constant contact between one side piston and the disk. You can check for this by looking down through the gap between the disk and the pads, squeese the brakes and check if both side pistons move, or if its only one. You can (apparently) fix the problem temporarily by taking out the pads, pulling the pistons out and cleaning and greasing the sides of the piston. Then putting everything back together. I had this problem on my Hayes 9's (sticky piston) but it was so damaged by the time I realised what the problem was that I could not fix it.