Haha!
All this thread could ever possibly be is a bunch of speculation, as no one on here knows Brad Pitt, or knows anyone that knows him, and if Brad is smart (and I think he is), he wouldn't tell a single person a damn thing about his use of anabolic compounds.
But my speculation is that for Fight Club, he was not on any steroids at all, having watched that movie several times, I see absolutely nothing of note about his physique that would indicate steroid use.
Now, for the movie Troy, I think he ran something to pack on the muscle for that film, there is quite a significant difference in musculature between Troy, and any other film Pitt has ever been in.
That being said, I think it is POSSIBLE he put on that muscle all-natural, but I just don't think thats the case. He was 40 years old when they were filming Troy, and even men with REALLY good genes don't generally have the natural androgens to pack on that much fat-free, quality muscle like he did, not without pharmaceutical help.