Well that didn't take long.
It is getting harder and harder to keep a major entertainment release under wraps these days. Halo: Reach has leaked out to the Internet via Microsoft's own download servers. JoyStiq reports that a review copy was put up, intended for download only by vetted reviewers.
Somehow Game-Tuts managed to find a way to get around Microsoft's protection software. There is at least one video of in-game play out on YouTube already. The game doesn't appear to be leaked out for widespread use yet, but enough people have gained access to it to fill the Internet with spoilers.
Odds are, Reach is available on a few private torrent clients right now. That means it is probably only a matter of time before someone takes the risk of putting it somewhere relatively public. Even so, I don't expect Bungie is too put out over this. Halo: Reach (like every other Halo game) is going to sell like gangbusters, a review copy leak won't change that.