This past weekend, Windows 7 build 7000 leaked to torrent sites. It's not the first build of Windows 7 to leak, but it definitely is the most important one so far. You see, the first 7xxx build marks the entrance of Windows 7 into beta phase (6xxx builds were pre-beta builds). Microsoft planned to release this build to a select number of beta testers, as well as to MSDN and TechNet subscribers. Unfortunately for Microsoft, and fortunately for everyone who couldn't, but wanted to, get access to the beta, this led to a leaked build. The company pulled the release off of MSDN and TechNet, and also removed the "Windows 7 Beta" entry from the next quarterly update kit for those part of the Microsoft Action Pack Subscription (MAPS), which is slated to be sent out to subscribers on January 5, 2009. Thanks to Google's cache, you can see that Microsoft also referred to build 7000 as "Windows 7 Beta" on MSDN.
Full Story: The Windows 7 build 7000 leak and Microsoft's reaction
Full Story: The Windows 7 build 7000 leak and Microsoft's reaction