A group of modders calling themselves GameTuts learned the game was live (how they learned this probably warrants the most serious investigation), then managed to bypass Microsoft's precipitous price force-field. Code in hand, they hacked it for public play, leading initially to a few rogue YouTube videos of unrevealed multiplayer levels, but eventually--perhaps inexorably--to the code's appearance on file sharing sites.
http://www.pcworld.com/article/203974/microsoft_wields_banhammer_against_halo_reach_pirates.html?tk=hp_new
Before you assume this only applies to someone foolish enough to play the game while logged into LIVE and online, it's worth considering whether Microsoft can tell whether you've played the game--offline or on.
