As I am renting a car this week, I was glad to see Sixt rents the smart here in Berlin for about $110 USD for two days (optional insurance excluded). The next lowest cost car was a VW Polo for $160 USD for two days.
The catch: smart is 200 km/day free, whereas all other cars are unlimited km/day for free and I was going to be too close to that for comfort.
I ended up renting a VW Fox from Europcar for $140/2 days b/c I can pick it up in town and return it at the airport at no extra charge —and unlimited mileage.