No, the explosions and collapsing buildings are all baked animation. You can look at the animated models in the model viewer and see that they're all fully animated. A lot of death animations for characters, especially in L4D, are animation that transitions to physics after the animation is finished, though.
Hey Adam, we are looking at a baked RayFire (http://www.mirvadim.com) animation working inside the Unreal Engine. Almost worthless for gameplay but awesome for cinematics.
with baked animation I have complete control over the scene. With unreals fracturing (i think you mean kactors?) I cannot know that the giant piece of building will be in that exact spot at that exact time to blow up that helicopter or smash that APC. :)