It seems that gamers without an internet connection will not be allowed to play Half-Life 2! Here's a quote from an interview with Doug Lombardi:
Q. Will the boxed versions of HL2 require any sort of Steam installations?
Doug Lombardi: During installation the user will be prompted to authenticate the copy and create a Steam account. After that is complete, the single-player and LAN games do not require an Internet connection.
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