Miners have no incentive to employ monitoring of the blockchain via multiple clients like Eligius did to avoid mining any blocks on the invalid chain, if they're just going to be reimbursed anyway.
Except that in this case 0.8 miners were technically mining on a good chain and it was quite hard to detect that there was a problem. My pool has been always checking if it has the same blockchain as some other nodes, obviously this cannot cover all possible cases.