Sounds like repolymerized brake dust to me. What happens is that as brake dust settles on the wheels, and the wheels get hot from fast driving and/or hard braking, the glues in the dust (brake pad material is held together with special glues that are affected by heat) begin to remelt, and the dust collects in little, pinpoint size globules. Then the glue in the dust just sticks the tiny globules to the wheels.

The globules are very hard and very difficult to remove. The best remedy is to keep the wheels clean and well waxed so that the stuff doesn't stick. But if you have it, one thing I've used with some success is Oil Flo Safety Solvent & Degreaser. It's very agressive, cuts EVERYTHING (not the wheel paint, tho. And I make sure to keep it off the body paint, too.) from wax to road tar to bugs to contact glues, and WILL cut wheel weight glues if you're not careful to protect them.

Try a little on a rag wrapped around your finger and rub the spots hard. It may take patience, but the Oil Flow should work. For really stubborn spots, I've used a popsicle stick inside the rag, the whole thing wet with Oil Flow, and sometimes the stick itself. But you must be careful not to scratch the wheel with the stick.

I had this problem with a set of used wheels purchased unseen off of a 4.4 - from the guy's hard driving and careless wheel cleaning. After some time with the Oil Flow, the wheels came clean. Now, they are WELL WAXED, and I clean them regularly.

Oil Flo is available from Car Care Specialties for about $13 for a 16 oz. bottle. Order line is 877-796-8300.

Good luck!

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Sageriders
2011 328, Water [blue], Saddle
2006 X5 4.4i, Sterling Grey/Truffle, Premium, Sport, Cold, Rear, Adj. Ride Ht.
2005 X5 3.0, Sterling Grey/Black, Sport(Totalled!)
2002 330i, Grey-Green Met/Truffle Br. [Sold]