Steam client beta may see some interesting language alluding to game sharing in the "steamui_english" file of the latest update. It's suggested that Steam users will be able to set a shared game library – when the original owner logs into the game, the game sharer gets a prompt to quit out.
If you want to see the file for yourself, you'll need to be running the beta version of Steam – go into Steam settings and opt in, then restart the client. Then, you'll need to head into your public folder and open the "steamui_english" file in a text editor like Notepad. Final step: crack open a cold soda, sit back and feel good about being a computer genius.