What is stopping me from hacking the mining software such that if I do happen to get the right hash I keep it for myself, but if not then I just submit my work done and get my share of the mined income.
The only way I can think of to check this is for the pools to monitor the block chain and look for suspicious activity, but I'm not sure what this activity would like in practice and that wouldn't just be a higher than elevated rate of whatever they're looking at.
It seems to me that this would be especially attractive on pools like Eligius that do CPPSRB and even more attractive the higher my hash rate as then I have a high chance of getting blocks all to myself.