Quote Originally Posted by http://www.planethalflife.com
Half-Life 2 pre-loading begins to Steam account holders today. Over the coming days, those who have selected the pre-loading option will have encrypted Half-Life 2 files delivered to their PC. No purchase is required to pre-load HL2, but is required to activate the game. Purchase options will be released soon. Then, the moment the game is made available, those who have pre-loaded and purchased Half-Life 2 via Steam will be authenticated and ready to start playing. For more information, please visit www.steampowered.com
It started yesterday, but the servers were so busy I didn't get a chance to start downloading until today. It's downloading as I type this. The downloads about 500MB. Unpacked, it's a little over 1GB.

Are any of you going to start pre-loading?