Halo: Reach leaked, massively, to all the major BitTorrent sites today. The music industry has struggled with huge pre-release leaks for years, but for the most part, film, television, and gaming properties tend to only hit BitTorrent after the initial release. That's why this is a big deal. On the other hand, there's one very important reason this is not a very big deal.
Halo: Reach is likely to be the biggest Xbox 360 game of the year, possibly the biggest game, period. It's the last Halo game to be developed by Bungie, the now-famous team that developed the first three Halo games (although not the last Halo game; Microsoft subsidiary 343 Games will take the reins for future titles in the series).
Evidently, Microsoft put the finished game up on the Xbox Live Marketplace, the same store that will sell the game to the general public when it's officially released on September 14th. The game was loosely hidden behind some security so it would be invisible to those browsing the Marketplace, and was given a price of 99,999 Microsoft Points (about $1,250) to further discourage piracy. But it remained in the Marketplace presumably so Microsoft could pass out copies to reviewers.
The security wasn't nearly enough. GameTuts, a console-modding site and community, somehow got ahold of the game (whether they paid for it isn't clear) and altered the code so it could be played by anyone with a somewhat difficult Xbox 360 modification called JTAG. Despite GameTuts's professed desire to keep the game private, it leaked to BitTorrent, where anybody can find it right now.
This sounds desperate for Microsoft and Bungie, but in fact it shouldn't be much of a problem. JTAG isn't an ordinary mod; it requires the user to solder a parallel port onto an Xbox 360, which is far more effort than most curious parties could muster. I suspect that most gamers will think about downloading the game, and then remember that it'll be released in a few weeks. Is it really worth taking a soldering iron to an Xbox?
Of course, that's not to downplay the fact that Microsoft really needs a better system for delivering early review units of software. But I certainly don't think the leak will impact sales--you'll be seeing this game everywhere next month, don't worry about that.