Valve must be really pissed right now but I'm wondering how bad this really is. OK, this sucks. This sucks for Valve and while I'm not too bothered about HL2, I've a lot of respect for those guys. Now, how will this impact things?
The CD-Key code is available so any cracker worth his salt can create any number of CD-Keys. Problem? No. This code is about a month old (or so someone slashdot said). The game is a couple of months away from release. No CD-Key that validates on the leaked code will work, that's beyond doubt.
Licensing - big money nowadays, although there is a lot of players at present (D3, UT Warfare, Lithtech, renderware, even Q3 and others). Still, big money. Will companies use it? Not in it's current form. If Valve get a sniff of copyright infringement, they'll be racing to their lawyers faster than the speed of sound because they'll want a high profile, big money victory (which they'd probably easily get) to disuade future attempts. What will happen is some developers will use ideas - graphical stuff, AI etc but nothing huge. Will they gain a lot from this leak? Yes. Will Valve loss a lot from this leak? I doubt it. I doubt anyone can take enough so that they'd decide against licensing Source (what an ironic name now) because of the legal issues. I'm sure HL3 will be considerably more advanced than HL2 so I can't see HL3 being hugely impacted. However, it will probably be somewhat impacted as this leak will bring a number of developers up to speed and some of them might stay up to speed. I doubt this will be significant compared to what HL2/HL3 will make Valve but still, it's something.
Warez - this is the source source, not the game. No data files = no game. Useless for gamers.
Cheating - this is the big one. This code leaked is at least 2 months away from the final code and builds with multiplayer cheats in will be useless. However, security through obscurity is a major part of gaming security as the server doesn't have the resources to do as much as we'd like. If people can see all around the source, that security will be gone. In the next month or 2 or 3 or 6 this will be changed as much as they can but I doubt they can change that much without delaying the game.
I think this will impact online cheating. It won't be huge and, frankly, I can't see online cheating in HL2 ever not being an issue so the damage is limited but that doesn't make it much better. I don't think this all sucks for Valve or for us as much as it would appear but it still sucks.