In addition to serving as DRM, Steamworks will provide auto-updating, achievements, matchmaking, downloadable content support "and more." As such, Civilization V will require an installation of the main Steam client.
In addition to the regular edition, Civilization V will be available from Steam in a 'Digital Deluxe Edition' packing the bonus 'Babylon' civilisation, a behind-the-scenes video and the game's soundtrack. Pre-ordering either edition through Steam will net you the downlodable 'Cradle of Civilization Map Pack: Mesopotamia' for free.
Boasting "an entirely new combat system" with ranged bombardment and now based on a hex grid, Civilization V is scheduled for release in September, according to Steam.
Pre-ordering right now.