Nope they won't, FIFA will simply refuse to go down that road. And in some ways, I can understand their position.... Take yesterdays "goal".... Obviously we can see that the ref made a mistake, but, the decision was made by the ref, the ball was immeadiately cleared up field, it was still "live" and in play - Lets say that when the ball went up field, the Germans had scored - So the first time after the controvesy that the ball went out of play, and the game stopped, was when Germany score... Do we then go back to a replay of the England "goal", check that it was, and allow that, so negating the German goal? In another scenario, it could have been two or three minutes before the ball went out of play, and "instant Replay" could be used.... Difficult...
Tennis and rugby, like American Football, give themselves to Instant Replay because of the Stop/Start nature of the games, as does cricket, for example. Because Football is a constant flowing game, it would be hard to apply the same form of replays to assist.....
Thats not to say that I don't believe that there could be better ways to ensure that errors like yesterdays could be eliminated, or Henry's Double Hand Ball that stopped my team reaching South Africa could be spotted..... But Instant Replay is not the way, IMHO....