I think oleganza is referring to the fact a new block should be generated roughly every 10 minutes, and until the transaction has been confirmed you can attempt a double-spend.

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.

10 minutes is a rough guideline on how long it takes the network to confirm a payment, and verify that a transaction is not a double-spend or otherwise invalid. For large payments you'd want to wait longer because a theoretical attacker would have more incentive and resources to construct a double-spend.