During an interview with a French TV station, Zinedine Zidane did not reveal exactly what Marco Materazzi said to him to provoke the headbutt, but he said that it was an insult to his mother and sister (read translated transcript here).
For a more complete, and very interesting, profile of Zidane the man, check out this article written in 2004:
Zidane's occasional violence may well be a product of this internal conflict: the French-Algerian who is for ever suspended between cultures. But it is equally likely that, although in public he presents a serene and smiling face, he is underneath it all every bit the same hard nut he had to be to survive the mean streets of La Castellane. 'Nobody knows if Zidane is an angel or demon,' says the rock singer Jean-Louis Murat, who is himself a fan of the player. 'He smiles like Saint Teresa and grimaces like a serial killer.'
And here's an email one of my coworkers sent that sums up how most soccer fans (except Italian ones) feel about The Headbutt:
On the one hand, Zidane lost his self-control, sold out his teammates, besmirched his legacy, reversed the game's momentum, ruined France's positive karma, destroyed a nation's hope for a World Cup, detracted from the French team's larger symbolism as a force against racism, broadened the cultural divide in an ethnically torn country, and sullied the reputation of the world's most popular sport.
On the other hand, he dropped that Italian guy like a bitch.
P.S. Don't worry, Roger, despite my recent obsession with Zidane I haven't forgotten about you. Congratulations on winning your fourth Wimbledon.