An observant user on the Neogaf forums, Grief.exe, noticed that there are a few lines of code in the latest Steam client beta that could indicate that game sharing may be on the horizon for Steam. The code includes reference to Shared game library and file access.
The code in the client update is as follows;
"SteamUI_JoinDialog_SharedLicense_Title" "Shared game library"
"SteamUI_JoinDialog_SharedLicenseLocked_OwnerText" "Just so you know, your games are currently in use by %borrower%. Playing now will send %borrower% a notice that it's time to quit."
"SteamUI_JoinDialog_SharedLicenseLocked_BorrowerText" "This shared game is currently unavailable. Please try again later or buy this game for your own library."
The code looks like users may be able to create libraries of games that they can share with people on their friends list, and the system will track when a game is in use (borrowed) by someone and not available for additional sharing.
Valve has not made any comments on this so we will have to wait and see what, if anything, becomes of it.