The Steam Users Forums (thanks Shacknews) announces that Valve called shenanigans on approximately 20,000 Steam user accounts yesterday, and has disabled access to them. Word is: "The method used was extremely easy for Valve to trace and confirm, and so there is no question that the accounts disabled were used to try and illegally obtain Half-Life 2." The post goes on to describe the circumstances under which Steam accounts will be revoked, as well as avenues to follow should you feel your account was banned erroneously. They also go on to debunk what has become the popular myth that they released a bug to the warez circuit to entrap pirates:

Valve did not put out any kind of fake key or fake warez or hack instructions to trap people. The hack came from the "community" as do they all.