Didnt the chef of police hire some "hooligans", to fire up the conflict so he could intervene drastically and show what a hardass and competent police chef he is?
Also, Ive heard numerous (credible) reports of police brutality and dubious stuff..
I think that the probability of infiltration is really low. After the events, public opinion and judges cracked really hard on the cops, who were brought to trial for every possible offense committed. The co-operation in disruption of the public peace, through infiltration, deliberate provocation etc, didn't ever reach even a first degree court of law. If there had been the faintest smudge of credibility, the trial would have been held.
Brutality, on the other hand - trials are being held as we speak, although probably most of the cops will be acquitted because one of the main pieces of evidence for the main brutality trial has disappeared a couple of months ago (or was never there, depending on who you listen to).
In any case, the death of the young protester Carlo Giuliani has nothing to do whatsoever with police brutality; a man attacked another man with a fire estinguisher, a weapon heavy enough to kill, and the other man responded with gunshots. As I said, maybe one can make case for police disorganization rather than brutality; if the car wasn't half-full of scared young draftees maybe they would have been able to avoid being cornered in the midst of assaulters, etc.
Anyway, don't believe everything you read on "alternative" sites on the G8 events. Don't even believe everything I say if you don't want to - be careful in general, the videos, photos etc have been manipulated to no end, and the testimonies of course are all more or less false. Not to mention that it was held in Italy, and we as a populace are normally inclined to hypothesizing evil plots to rule the world even behind, say, variations in the price of salami.