The ever-watchful NeoGAF has stumbled across evidence that Steam may be adding the ability to lend your games to other players. Can you imagine? User Grief.exe found the following lines in the steamui_english file:
"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."
Find this for yourself by updating to Steam Beta and going to Steam > Settings > Beta Participation and then looking for the aforementioned file in the /Steam/Public folder.
Such a feature sure would be nice. I'd no longer need to do the "offline" trick to let my sister check out my library.