I tested on the latest dev version: ecoCreature-0.2.2-SNAPSHOT.jar
I am using CraftBukkit 1.4.6-R0.1.

Console errors have been fixed, but heads still do not drop.

For testing purposes I made it so the player drops wool 100% as well. The wool drops but not the player's head. Here is the config:

Player:
Coin_Minimum: 5.0
Coin_Maximum: 10.5
Coin_Percent: 50.0
NoReward_Message: ''
Reward_Message: '&7You are awarded &6&7 for murdering &5&7.'
Penalty_Message: '&7You are penalized &6&7 for murdering &5&7.'
Drops:
- skull item:1-1:100
- wool:1-1:75

Thanks for the report. I'll look into it. Hopefully, it's something super simple.