Bitcoin is not reversible after 10 minutes, credit card transaction is not reversible after 90 days in some cases. Guess how much fraud will happen to anyone selling Bitcoin with PayPal, Visa or similarly reversible payment mechanism.
The pain in the ass with buying Bitcoins is due to a simple thing: you never own government currency, you always ask someone else for permission to use it and it's always based on human decision. Gold and Bitcoin you can own. Paper bills you can own to some degree (they print more of them every minute, thus taxing your savings all the time). Digital virtual currency USD or EUR in your virtual bank account - you don't own. Banks can even reverse SWIFT/SEPA transfers if they want.
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.
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.
