According to Neogaf, a developer build of Gears of War 3 has been leaked onto torrent sites. The leaked version allegedly includes both the full single-player campaign and multiplayer modes.
As a result, video like this one, which shows the “beta” loading screen and a few spoilerific minutes of the campaign, have been popping up all over youtube and other video sites. Lots of information about the game and its story are now out in the open: If you don’t want Gears 3 spoiled for you, then I’d suggest you avoid looking for information about the game outside of official channels.
Since it’s the leaked version of the game was intended for use by developers only, (It’s A JTAG version, if you know what that means), the game can only run on a modified console or an Xbox 360 debug unit.