The fact is Materazzi is well known for being a butcher during the "calcio italiano".
He will deny his words but Zidane is not the guy to act violently for nothing (by the way, remember Zidane reading the text against racism before one match and keep in mind that he is from Algeria), so there was some hard talking and the final + stress made Zidane acts this way.
If you watch carefully the sequence in slow, you'll see Zidane walking away with a smile (like "well, let's forget this and leave me alone") after having Materazzi on his shoulders and arms... and as he's away, Materazzi talks a little more, it's this sentence that made Zidane acting.
So, this stuff just reveals the evidence : football is full of racism and verbal violence. And I'm sure that the italian's words were very very sin... I just hope this story will help to fight this more.