An official update from Valve:

The second phase of the Half-Life 2 preload begins to Steam account holders today.

This will allow users to download the majority of the audio of Half-Life 2 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 Half-Life 2 via Steam will be ready to start playing right away.

Recent developments:
Half-Life 2 pre-load begins
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