I've heard a number of different scenario's regarding what Materazzi said to Zidane to provoke such a reaction, none of them good:
"according to the bbc, (they used a lip reader ) Materazi wanted an ugly painful death for zidanes family
now that may not sound bad till you realise Zidanes mother is severely ill in hospital"
This courtesy of a French news agency;
Zidane holding his shirt says to to Materazzi:
ZZ: "Ordinanza de tirare il costume!!" (stop pulling on my shirt!!)
MM": "Taciti, enculo, hai solamente cio che merite..." (shut up f***er, you only get what you deserve)
ZZ: "si e cio..." (yes, sure...)
While Zidane starts to move away, Materazzi goes:
MM: "meritate tutti cio, voi gli enculato di musulmani, sporchi terroristici" (all of you deserve that, f***ing Muslims, terrorist *******s)
edit: Here's a link to more Materazzi golden moments:
http://www.youtube.com/watch?v=rKGcUr0S-FU