Steam should have an option for those downloading to opt into a torrent-like system (through the steam client, of course). You offer up your upload, you get more download from other downloaders who are uploading. When a few million people are trying to download at once, the users could actually take some of the strain off Valve's servers. Of course, I'm sure implementing something like that would be tremendously complicated and probably have ugly security issues (I'm largely ignorant to the workings of torrent systems, so I wouldn't know), but it would be a nicely self-balancing system if it could be pulled off.