Our 12yo twins took the SAT a couple of months ago. We registered through a talent search (NUMATS), although I've heard you can call College Board to register directly with them. Our kids each took a form they were provided (by NUMATS?) in lieu of a photo ID -- it had their signature and a parent signature (both signed in advance), and they had to sign again in the presence of the proctor. If you register by phone with College Board, I am sure they can give you the low down on the procedure.
Another interesting tidbit -- when we arrived at the testing center (a huge high school), parents were not allowed past the building vestibule. The testing room numbers were posted there, and the kids were on their own to follow the arrows to find their testing room. There were no staff at the entrance to answer questions.
