It's reported that the exposure you're getting from a single scan is equivalent to the total radiation you receive from flying at 30,000 feet for 3 minutes. So... practically speaking it's not a concern.
The real concern is if the machine malfunctions. That kind of thing may be rare to impossible. But it's happened before with hospital equipment.
The real danger - if there's any - would be to the airport worker who stands near the machine all day long without any protective equipment. When you see them begin to wear radiation exposure badges (been there, done that on a previous job), well then we travelers can begin to talk about exposure.
Personally I prefer that over the new regulation pat-down, unless of course she's cute. Somehow though I'm not expecting a "Girls of TSA" special in Playboy any time soon.