You know SavageD, I never thought about that. If something were to happen to Valve, or western civilization collapses or what not (conspiracy theorists, get your pencils ready) then exactly how would you play HL2 again? Silly, I know...but it is something I haven't considered.
I would imagine they would do something along the lines of what ID did with Quake 2 and the no-CD patch. Quake 2 was one of the first games I remember that required the CD in the drive...something I didn't like since I enjoyed playing Q1 with Motley Crue's decade of decadence album in the background. It wasn't until a little over a year later, when the hype and the sales died down that they released a patch that removed the CD requirement.
I imagine that, maybe a few years down the road, Valve will release a patch that removes the need to authenticate the game. After they make their money from the original retail version, release an Xbox version, a "GOTY" version, and possibly another release with HL2 as part of a Valve trilogy boxed set or something, I don't see why they would care if it was pirated at that point. That's pretty much how ID and Blizzard have handled their own games.