The fAQ says:
"Note: some Steam-enabled single-player games (such as Half-Life 2) purchased at a retail store will allow offline play until the user decides to play online or enable other Steam functionality. From that point on, an Internet connection will be required."
So don't worry about HL2 SP without internet connection, *unless* they mean that, once you used Steam to update your game, it'll authenticate afterwards. But I think they are refering to the case where you add MP via Steam to a previously SP only version.