HL2 will be unique in that a WON server was never required when you're not playing a multiplayer game. STEAM is required. Again, Doom 3 doesn't require a central server to play single player, ever.
You can get CS:CZ in a store and install it. It makes a seperate installation that isn't Steam. It doesn't require Steam to run. It doesn't connect to the internet at all. To play online, you have to install the game into Steam, which takes the resources of that single player version and can either leave them (as they are now just duplicate) or can delete them. If you leave them, both versions work (although the single player is unpatched).
It is safe to assume HL2 will work the same way. Therefore, you CAN play HL2 single player if you're so damn worried about magical elves stealing your porn off your harddrive through Steam.
The fact of the matter is, most things people think about Steam are total misrepresentations. It doesn't get file names and store them in a GCF, that's because it sets a file that is a certain size and doesn't zero it out (hence old data can be in it, but will be overwritten). It can start in "offline" mode for LAN play. Etc, etc, etc.
Also remember, since you people keep bringing up Blizzard and Battle.net, that Blizzard got sued for having battle.net get the user's email out of outlook express (or something to that extent) when someone tried to use an invalid or pirated CD Key. They lost. You guys keep saying "oh blizzard this and that and look now valve is eviiiiil" but Blizzard is in fact the one who has already gotten busted for breaking into someone's privacy.