Over on Half-Life 2 Files there is an article about a pirated warez version of Half-Life 2, released via the Bittorrent netowrk. This user sent word of this to Gabe Newell @ Valve and he had this to say:
We’re running a bit of an experiment. We’re keeping track of the accounts that do this and will be shutting them off.
Basically Valve released a “warez” version to flush out the leechers, and in essence banning them from Steam. Therefore, you should probably just buy it, like everyone else. : )