While the good people at Valve have already decided that your bandwidth is their bandwidth and that you're a beta tester wether you like it or not, GFK's original CZ preload post will (so I have been lead to believe) work for CS:S.
Just look for references to 'Counter Strike source' rather than 'Condition Zero' and remember that you're utterly on your own if you play about with these files.
Thread (http://forum.deadmen.co.uk/index.php?showtopic=5124)
This release is NOT an early release of Counter-Strike Source. It is a beta designed to test compatability (i.e. Valve are not sure if it will blow up or not). When the beta test is at an end the download will be deleted and you'll not have access to it anymore. At a later date we can all download the CS :Source final release. You may well be chuffed to have a look at it now but if you don't fancy the idea then at least you can do something about it. Having the ability to decline a beta download (or any download for that matter) via a pop-up box or whatever would be nice. But remember it's Valve we're talking about here, so fooling Steam seems the only way to take back control of your PC.
have Fun :D
Remember, kids. Fight the power!
But i want to be a Guinni Pig :D
Then off you go little one, and play to your hearts content :D