This is neat:
The Impressioner consists of a sensor that goes into the lock and sends information back to a computer via USB about the location of the lock’s tumblers—a corresponding computer program comes up with the code, depending on the make of car you’ve entered beforehand. Once you know the code, a key-cutting machine can use it to carve up a key.
Right now, it’s a prototype that only works on Ford car locks. The article points out that both locksmiths and thieves can use this device.
EDITED TO ADD (2/16): How it likely works.