A thread on NeoGAF notes some code in the new Steam beta suggesting Valve is planning to allow Steam users to select games to share with a circle of friends. This is confirmed on Kotaku, where they say Valve has not yet responded to the report (not a shock since their story went up at 3:00 am on the west coast). Here is the code in question:

"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."