Steam has just announced that users will be able to share their games between authorized computers.
QuoteOnce a device is authorized, the lender's library of Steam games becomes available for others on the machine to access, download, and play. Though simultaneous usage of an account’s library is not allowed, the lender may always access and play his games at any time. If he decides to start playing when a friend is borrowing one of his games, the friend will be given a few minutes to either purchase the game or quit playing.
http://store.steampowered.com/news/11436/
Basically this is how Steam works now but you'll be able to have separate users on the account who can all have their own achievements and save games.