since it was leaked like this, competitors cannot use it. if valve suspects they are, they can attempt to reverse engineer the other guys tools, find some kind of proof, and then kill them in court.
the real problem is that hacks/cheats have access to the source. they've never had that before. any ideas about how much easier it is to find an exploit, when you have the source right in front of you? hell, write a few test programs, and they will find the exploits for you
i imagine the engine for hl2 will still be quite licenseable.