The article at this link is a bit funny, but if it is true that Materazzi made racial slurs against Zidane, then his headbutt was the ONLY proper answer to that.
Racism on the pitch should not be tolerated under any circumstances, and a healthy team would not tolerate racist remarks from any team member.
If Zidane's reaction was a response to racist remarks, then his headbutt is a symbol for a world cup that did not tolerate racism, and that united people from all over the world instead of dividing them.
On a side note, I am very happy for all the Italians :-) and I'd like to thank my Italian neighbours for having invited us to their place to watch the final.
Enough football, now back to work.