Make blog has a great video of the Xfinger. It's a prosthetic device that allows partial finger amputees to regain grip. My first thought was whether or not you could flip someone off--not particularly insightful. After watching the mechanism in action, they show it after a silicon, finger-shaped, sheath is covering it. I was disappointed that the action was hidden.
It reminded me of an encounter I had with my daughter. My oldest was about six. We were at a craft store when a man with a prosthetic leg walked into the store. My first thought was whether I could maneuver us out of the way so she couldn't see it. Not because I didn't think she could handle the idea of the sight, but because, as parents know, kids can say the darnedest things--and I didn't want to contribute anything that might make the guy uncomfortable. But, no, there was no place to go. She saw him walk by and said, 'Mom, that guy had a robot leg. Cool!'
UPDATE 7/2/07: The XFinger has been profiled in Wired Magazine.