Totti was labeled dirty not for the one incident, but based on a long record of aggressive behavior. Hell, even in this WC he came close to expulsion for his 'combative' style (I think it was in the US match).
That's not the point. DP retaliates too, just not to the same degree. I've often seen him make tackles that could not be misinterpreted as clean, bad temper, complaining about decisions etc. Zidane, when he retaliates, can be a lot more violent. That's the difference. It's not the case that DP is the embodiment of integrity while Zidane uses every trick in the book to bend the rules.
Look, Zidane's record of 12 expulsions is an ugly one, and for all the bookings and incidents over the years, condemning him for it is good and fair. Not only that, the jokes over the head butting were funny too. But to put him in the same bracket as _Materazzi_, no in fact saying that he's more dirty, is just the most ridiculous thing ever. Yes, Matrix may not have collected as many red cards, but that's because he's a sly mofo, he's expert at getting away with stuff. Zidane, meanwhile, prone to losing is head, does not in any way intimidate or provoke opponents. How you can equate those two is insane.
Del Piero has 2. So by this logic, he is one expulsion away from being as bad as Materazzi.