Fallout New Vegas will use Valve's Steamworks DRM, meaning that players will only be able to access the game through Steam.
The Steamworks SDK will also be utilised for achievements according to Bethesda senior producer Jason Bergman:
"Yes. Fallout: New Vegas will fully utilize the Steamworks SDK," he answered in a Q&A on the Bethesda forums.
"This means that retail PC copies will activate via Steam. We are also using Steam for achievements and other features (but not multiplayer, of course. FNV remains a single player only game)."
We brought you a massive string of Fallout: New Vegas details back in February. Have a look if you want to know what's what.