Mike: It's not just Smart's that have that issue... it's any car that has disc brakes pretty much.... unless maybe they're drilled and/or slotted... but either way i think there'll still be a brake dust issue. it's just how the brakes are. there's nothing covering the brake pad nor the brake disc, so whatever is worn off during braking is either blown out either side, or sticks to the rim somewhere.
The easiest method i found of keeping the wheels clean and brake dust free.... buying Kleen Wheels brake dust shields. it's basically a thin piece of aluminum with vented slots to move some air past the brakes from the spoked side of the wheel while preventing dust from coming back. You'd unbolt the wheel, pop the Kleen Wheel in place, making sure the holes line up (and you also may need some "gentle persuasion" to get it to fit nicely around the center-mounted wheel weights), bolt the wheel back on, and spin the wheel to make sure that nothing rubs or hits something. What i meant by "gentle persuasion" was that i actually bent a small area of the shield where the wheel weights would have hit, put the shield in place, pushed that bent part back, and used a hammer to very gently tap it in place so it fit snugly around the weights. doesn't take much to do, since the metal is so thin, but don't want to hit it too hard that you crack or break the weights off or something.
Think i've had mine on for like two or three weeks now, and i don't think my front wheels look any more dirty than my rear ones.... and i don't think any of the dirt is brake related.