As seen over at the Steam Powered News Page.

"The sixth phase of the Half-Life 2 preload begins to Steam account holders today. This will allow users to download the Half-Life 2 maps in encrypted form. No purchase is required to pre-load Half-Life 2, but is required to activate the game. The moment the game is made available, those who have pre-loaded and purchased the game via Steam will be ready to start playing."