I'll second the sock thing. I had a couple dozen while sports socks that were starting to wear out. So i just bought all new socks and resigned the old ones to cycle/car cleaning duties. They work great worn on your hand because they give you the ability to get in there among the spokes and other tight places with a finger or 2. May not seem like much, but once you try it you won't wanna use plain old rags anymore. Bikes as you know are nothing more than a huge collection of nooks and crannies, perfect for socks and makes it all go by much easier and quicker. Heck, if i hadn't had a drawer full of old socks i'd have bought new ones just for this purpose !
As for the original question, i haven't tried it but some product lines like Mothers make a cleaner especially for spray on removal of brake dust. How well it works i can't say, but they do make it.
