A level of "DRM" will also be used by limit installation. Dustin explained "you need to connect once to install the game." While singleplayer will be available offline, installation must have an internet connection in order to proceed. You are also required to have or signup for a Battle.net account in order to install the game. All achievements and friends lists etc will be available as soon as you logon, but the actual single player game is available anyway.
"You can [play single player offline], but we don't encourage it." Browder said. "We totally allow it if you want to do it," but the point is "you don't get access a lot of the stuff."