I know that in the movie they say "airplanes were a perfect delivery system", but.... yeah that doesn't actually seem plausible.
Pitt's character times the change from human to zombie as ten seconds, and the longest time mentioned in the movie is 10 min when he's at the Korean military base. Given that it has such a short incubation time how would it go global? I can't see zombies getting on planes in any situation short of the chaos happening in Israel. And even then the whole plane would change so quickly I think that any plane they did get on would go down before getting to its destination.
So again, how would the zombie virus - with an incubation time of 10 min or less (mostly shown as only 10 seconds!) - ever go global?