With today's Steam client update, the Linux version now has support for 64-bit games. Previously, only 32-bit versions of games were distributed.
Other Linux changes in today's update include a fix for finding Steam client skins, a crash when looking at community images, the Linux Steam overlay not restoring the proper mouse cursor, and a resource leak leading to graphics corruption. This update also has Big Picture improvements and other general changes and fixes.
More details on today's Steam update can be found via this SteamPowered.com news entry.