Not much. Zero-confirmation transactions are quite hard to revert, so they are widely accepted for small instant purchases.
Here's how you normally do it: send a transaction with fairly recent inputs with a zero mining fee. If you are lucky, it will propagate slowly and will reach the merchant sooner than 50% of the network nodes see it. When the merchant sees your transaction, he ships you the product, then you immediately send out double-spending transaction with standard mining fees (sent to your own address). It will propagate much faster and probably will end up mined before the first one.
Fraud with zero-confirmed transactions is most likely in some fully-automated schemes like lotteries, where repeated attempts to reverse the payment pay off immediately.
Here's how you normally do it: send a transaction with fairly recent inputs with a zero mining fee. If you are lucky, it will propagate slowly and will reach the merchant sooner than 50% of the network nodes see it. When the merchant sees your transaction, he ships you the product, then you immediately send out double-spending transaction with standard mining fees (sent to your own address). It will propagate much faster and probably will end up mined before the first one.
Fraud with zero-confirmed transactions is most likely in some fully-automated schemes like lotteries, where repeated attempts to reverse the payment pay off immediately.