Yes, they still exist.

Was re-watching the 2008 Blizzcon B-roll, and I noticed something strange. http://www.youtube.com/watch?v=O-rNgwWD1qU Skip to 1:17. The monsters are all dead, but the Witch Doctor levels up 2 full seconds after the last enemy is killed. This could mean 2 things. The first is that you gain XP for picking up Health Globes, which is unlikely but would be interesting. The other is that it takes that long to resolve any bonus xp from combos.

---

Q u o t e:
You gain bonus xp when you kill several enemies in rapid succession, or kill several enemies with one attack. It was probably the former, which would wait for a few seconds to see if you killed any more demons before granting the XP.

---

This is very likely the explanation. It seems about the right amount of time before the combo XP is awarded.