Steam Beta Linux client will enter in closed beta testing this month. About 1,000 users will have access to Steam client and one Linux game.
Though a date for full public release is yet to be announced, Valve is already adding new Linux games to its database. You can check out list of these Linux games from here.
More games are continuously added to this list and hopefully we will have good library of games when final Steam Linux client is released. Steam for Mac launched with 50 titles in March 2010.