As we reported months ago, there have been new indications on such a service already in June which weren't accessible for public by then. First attempts in the course of beta phase will start in the middle of September. In order to register yourselves for the beta, you will only have to join the official Steam group and hope to get accepted.
First uncertainties have been clarified in Steam's Q&A - the maximum limit of connected accounts, which are able to borrow games, is set to 10. While some games with required accounts or subscriptions can not be shared, it's possible to lend out all other games at one time. Furthermore it's explained, that a game can not be played by two users at the same time. If the original owner would like to start a game which is currently being shared and played, his or her friend is sent a message to close or to buy that game.
What do you think about this feature?