SketchFactor, a Bungie employee, has just responded to the rumors going around that Halo 3 might be at some retailers already. According to him, only a few people have the final copies of the game and are not playing it on live.
"No actual copies are avaialble to any retailers yet. The photos that surfaced earlier were EMPTY promotional/pre-order copies of the game. Many retailers have had these for a while now. It was a hoax."