[Pierre-Marie Baty]

Yes, Steam has been recently updated to allow offline playing. To play offline with Steam there is a new option in the Steam main menu, "exit" (rather than "exit and disconnect"). If you exit Steam this way (without disconnecting) your SteamID and the tickets for your current Steam session will be stored on your disk and you will be able to disconnect from the Internet and play offline.

The only drawback is that you need Internet access at least once for your Steam client to register and get a Steam ticket. Sierra must be loving this 9_9