Okay, based on what I've read here and on the Steam forums here is a list of the steps I think a person needs to perform to pay 'n play HL2 (on Day #1) using Steam and end up with a backup archive of the game. Feel free to tear my list apart.
1) Download Steam
2) Install Steam
3) Purchase HL2
4) Download HL2 via Steam
5) Install HL2
6) Backup SteamApps folder to CD/DVD
7) [Delete HL2 files from SteamApps folder *see below*]
8) Play HL2
* Step 7: This is the step I thought might raise some eyebrows. I know the SteamApps folder contains the installation data and is functionally equivalent to install CDs so the obvious conclusion is the game gets installed to a different location, right? Basically, that means HL2 is on the HDD twice, right? Yes, HDD are cheap but do I really want/need to have data for a game I already have installed still lying around on my drive. Anybody? I know. I should probably go post this on the Steam Users Forum.
"Worry is only anxiety over something that may never happen." -V2