Zidane appeared on French TV last night and apologized for his actions, but also said that he did not regret them. He was not definitive in what insults were directed at him, but stated that they were 'very offensive, very personal' and that they were repeated several times.
And it has been shown that Materazzi twisted Zidane's nipple moments before producing his insults and receiving the knock-down. Bizarre, but it does make the head-butt more understandable. Few things are as painful and infuriating as a nipple twist.
***UPDATE*** Here is another story including info from a second interview ZZ did. VERY interesting that he focused on racism. FIFA is already investigating the incident and they stated before the tournament that any racist remarks would be grounds for sending off.