Joe that's an excellent point about making your own install CD. Are you looking for a means to just install or backup the game? I mean if you could just copy the game directory to another external drive (or burn it to a DVD, since it's GB's in size) would that suffice, assuming it worked when you copied it back? Or do you want an actual install CD program?
Either way, you should post this on the Steam forum. It's a very legitimate concern, considering the size of the game, and it would be interesting to see how Valve responds. I can't see why they can't create a utility program that would package up the original game and the update patches in a single (be it large) install file which you could reuse later. However, backing up to a CD, you'd probably have to use another CD burning utility that would allow you to break it apart between multiple CD's (which some CD burning software already has). I'm assuming it would fit easily on a single DVD though (based upon compression of the files inside the install file).