As I said in a previous thread, it's NOT that simple and it's not something that you can put a chip in a football to solve.
The problem (with spotting the ball, anyway) is that not only do you have to watch where the ball is, but also when the player's knee hits the turf. If you put a chip in the ball, how is it going to know when the player's knee is down? It won't, it will just be able to tell how far forward it went, even if the player moved up after being tackled. It's not hockey, where it could be argued a chip in the puck could tell if it's in the goal or not.
And if you think it's easy to see where the ball should be spotted on every play, go out and ref a high school game. It's not easy, it's hard.
And for those who want to know how the first down line is "painted" on the field:
http://entertainment.howstuffworks.com/first-down-line.htm
CW