Unfortunately, this is something that's happened before. Actually, modders were able to get their hands on both Halo 2 and Halo 3 before their official releases. Individuals over at Game-Tut.com, the new title from Bungie Studios has been yanked from Microsoft's servers. While the technicalities are all their own, we imagine that it was made much simpler by the fact that Microsoft put the game on their servers, enabling a $1,250 download of the title for reviewers just a few days ago.
The game, which has been on the servers for a little while now, was originally meant for reviewers of the title, and was supposed to only be able to be downloaded courtesy of a special code. The modders were able to find their way around that, and have been able to download the title at their will. Game spoilers are popping up all over the place, even Bungie's official forums, and it doesn't look like it's going to stop any time soon. Unless Microsoft figures out a way to put a stop to it, and pronto.
Considering the title isn't meant to be released until September 14th, this is a pretty big leak for both Microsoft and Bungie. Whether or not it will make a significant impact on their sales remains to be seen, but we really doubt that it will. Halo: Reach is the culmination of the series, and while it may not have its prodigal main character that Halo fans have grown to love, it's still Bungie's swan song to the Halo Universe. And that means it will probably sell a ridiculous amount. Even with a leak like this.