Italia Wins the World Cup: What the Hell Happened to Zizou?

Seriously, does anyone know what pissed Zidane off so thoroughly that the did this? Did Materazzi say something heinous? What? It's hard to make sense of this action given how seasoned of a player Zidane is. One of my friends actually switched alliances to Italy after seeing that unprofessional behavior. Still, I have to wonder why when the game was still 1-1 that Zidane would do this, get red carded and then walk off the field. A sad way to end his career indeed.

As for Cannavaro taking his World Cup to bed, boy what I would've given to be that trophy for one night!

UPDATE: Guardian speculates that it was a racist slur. If so, what do you think?
UPDATE #2: Lindsay switched alliances too after Zidane's headbutt.
UPDATE #3: Le Monde considers different rumors about what Materazzi said, e.g. that he called Zidane a terrorist (this article is in French).