The "1st & Ten" technology was developed by a company named Sportvision. Sportvision has been selling the service (at $25-30,000 per game) since 1998. They also created the "K-Zone" used with ESPN baseball. Before their "1st & Ten" fame, they were known for the electronic "glowing" hockey puck and for digitally replacing signs in stadiums in real time for TV. (I understand that's still big business for them.)
For in depth info about how they do that and, check out
http://www.sportvision.com/
Or if you just want to learn about the "1st & Ten" Technology, you can find that info at:
http://entertainment.howstuffworks.com/first-down-line.htm
