Looks like there are some unlucky Halo 3 users out there, who got a scratched Halo 3 disc from Microsoft.
The Halo 3 Limited Edition Game Disc and Essentials Disc are apparently prone to scratches by the packaging. Microsoft just issued a free replacement program for those Halo 3 discs that is running until the end of the year.The Halo 3 Limited Edition sells for $69.99. There is no word yet about how many of these Halo 3 discs are affected. In the meantime I would just buy the standard Halo 3.
Microsoft just launched their most important Xbox 360 game on Monday.
More details about the Xbox 360 Halo 3 disc replacement can be found here.