I don't believe they will, I think they replaced that with ragdoll. However, it wouldn't be hard to spawn a bunch of physics enabled player-made gibs into the death code, I don't think.
Oh yeah, and no one knows for sure except Valve people
I'm pretty sure it'll be modable for those cases (such as the ant lion) where gibbing is useful. The zombies sorta gib by the looks of it, as in legs/torso and apparantly the headcrab itself detatching. So gibbing should be possible.