A Battle.net user recently asked about whether deaths in Diablo 3 were hand-crafted or not and Bashiok had this to say:
"I think the videos Steel linked explain it pretty well. We do have both havok based and hand-crafted animations, and which of these is used is generally dependent on which monster it is. We have a few in there, like the skeletal summoner and grotesque, that have hand-made death animations and those will play through every time. But most monster death is determined by how they died. Was it a critical, was it a critical from a type of elemental damage, was a specific ability used that maybe kills them in a specific way, etc. etc."
And when a user asked the follow-up question, "can we hope for dismemberment in Diablo III?" Bashiok
"Does decapitation count?"
Yes, I believe it does. And having played Diablo 3, I do recall seeing quite a few heads a rollin'!