From personal experience in beta (don't know if it still holds true), mobs that don't give xp will never drop chests. Never seen a single chest from a grey my entire time in beta. Grey mobs do still give normal loot however, ie meat, spit, skin, etc.
Edit: haven't been able to play yet so they may have changed it. Anyone that can post confirmed that greys will drop chests when mob is grey to solo/entire group?
Edited, Thu Nov 11 05:24:12 2004 by terdap