Here’s an official letter from Gabe Newell! Please read it.

Hello,

Recently, your Steam account was erroneously banned from Call of Duty: Modern Warfare 2.

This was our mistake, and I apologize for any frustration or angst it may have caused you.

The problem was that Steam would fail a signature check between the disk version of a DLL and a latent memory version. This was caused by a combination of conditions occurring while Steam was updating the disk image of a game. This wasn’t a game-specific mistake. Steam allows us to manage and reverse these erroneous bans (about 12,000 erroneous bans over two weeks).

We have reversed the ban, restoring your access to the game. In addition, we have given you a free copy of Left 4 Dead 2 to give as a gift on Steam, plus a free copy for yourself if you didn’t already own the game.

 

To share your extra copy of Left 4 Dead 2 with a friend, you can ‘Manage Gifts and Guest Passes’ from the ‘Games’ Menu in Steam, or visit this article on the Steam Support site for detailed instructions: https://support.steampowered.com/kb_article.php?ref=4502-TPJL-2656.

To access your own copy of Left 4 Dead 2, visit your library of games in Steam. If you didn’t already own the game, it will now be listed among your others there, and is available for download immediately.

Regards,
Gabe Newell
President, Valve

Source: Steam Forums (or if it’s not enough for you Kotaku)