I'm not sure whether this would be allowed. In soccer, for example, using earphones and so on was forbid based on the rule that it wasn't part of the allowed equipment.
I believe creaghpatr was talking about American Football. The coaches already use radios which have speakers inside the quarterback's helmet to relay the plays to them. It seems like this would be a logical jump as they could get more information and crowd noise would not be a factor.
:) yes, I thought so, that's why I used soccer to make the distinction... but I didn't knew football allowed such intense communication during the game, and it would be funny to see a quarterback trying to read his pulse while he's being tackled.