The Hushed Casket has details on how some people with access to the Epsilon build of Halo 3 got duped into offering access to the game to unauthorized persons by getting access to those users' gamertags on their own consoles, and using that gamertag to download the build.

Check the story for full details; there are no real plot spoilers in the story itself, but there are fuzzy image captures from both multiplayer action and campaign play from Tsavo Highway.