After the very controversial refereeing errors at the Argentina-Mexico and the Germany-England quarter final matches of the 2010 Football world cup, if FIFA does bow to media pressure and think about introducing technology to assist the referee and linesmen they should definitely not do it during the world cup. There is nothing like a high stakes failure to kill technological advantages as computers have never been allowed anywhere near the same leeway as humans have.
The best way to introduce the new tech would be first at a 2nd or 1st division league game and if passing muster then at a series of international friendly fixtures (and no, there this no such thing as a friendly fixture between say Brazil-Argentina or England-Germany) and pending some time to get the glitches out of the tech and the human operators enough time to get used to it, maybe at the next world cup.
PS- Even though I am an avid Argentina supporter I'd say that Tevez's offside goal was the worse faux pas as it really killed hot-blooded Mexico's pretty decent chances. IMHO England, except Gerrard, were never there against a much livelier Germany.