From what I understand, and for those that have seen the demos from E3, the physics and muscle/skeletal systems are the real innovations in the engine. To date, no other engine has had the spohistication that the HL2 engine possesses in combining these aspects in the gameplay. I would almost guarantee that anyone possessing the source would find the time to understand how Valve did this if they really wanted to copy their techniques. While I don’t agree that technology is a real differentiator for gameplay today, their stuff looks DAMN cool.
-ChrisM