A supposedly leaked Halo 3 storyline from Bungie has surfaced and can be downloaded here (.doc format). The file contains three documents, detailing 8 levels of Halo 3, including storyline and dialogue. From the prologue:

Upon returning from deep space, after destroying the weapon known as Halo during a conflict with the Convenant, you, the Master Chief and your trusty AI guidance system Cortana, find yourself in front row seats to the end of Earth. Covenant Armada Battleships attack at the Earth mercilessly in hopes of destroying your very world. Seeing the imminent danger, you jump into action. Literally.

There is a possibility of the document being completely bogus, but the file info shows that it was created September 14, 2005 by “citistreet”.