a legitemate worry.
I don't think this is going to be a problem though.
They reckon that hl2 will start pre-caching invsibly when it goes gold and by the time they launch you should already have it downloaded!
This worked fine with the hl2 movies. Whether they can pull it off with hl2 or not will be interesting.
You have to bear in mind that the only people that will be downlaoding it will be the people who have paid for it through steam.