This has been causing a pretty big ripple in the modding community, because Valve are using Alien Swarm to release Source SDK, their comeback to the UDK (Unreal Development Kit; allows you to create games using the Unreal engine, but without access to the resources a game studio would have and a few other restrictions). It looks like it'll allow easy map creation and release of mods that won't require games like HL2 to run, however the SDK alone won't allow access to the standard HL2 resources etc; any developer would have to create pretty much the entire game from scratch.
I'm not too bothered about the game itself, but I'll probably check it out anyway.