But that's why you can pre-load the game right now. I already have the game pre-loaded on my system (and it didn't cost anything). You do NOT have to pre-order the game to pre-LOAD it. You only need a valid steam ID (which is free). Then you can pre-order (or even wait and just purchase it online the day of release and then the pre-load will be unlocked.
IMO pre-loads aren't the issue. The traffic on the content servers != traffic on the authorization servers. They are different servers, as I'm sure you already know. I want to see what happens when all the HL2 users attempt to receive the required authorization to play the single player game at the same time. I expect it'll be similar to what already happened where people who were authorized prior to the authorization servers going down were able to play, while those that hadn't been authorized yet were SOL until the authorization servers were able to come back up. Remember, we're talking about a single player game here and I expect the number of 'online' users on "release" day to far exceed anything any MMORPG has had in the past. In other words, I guarantee you they are going to initially underestimate the required servers to authorize everyone.