0/conf will work for retail transactions. I bet the rate of people using a double-spend technique to screw people out of money would be roughly equivalent to the rate of people using counterfeit bills for a cash transaction. And whoever the retailer is can use security cameras or other techniques to track the perp down and prosecute them.
...unless you're buying something expensive, like a big-screen TV.
Are you really willing to risk jailtime to try and steal a couple grand from a retailer who probably has security cameras aimed at your face?
Besides, retailers expect a certain percentage of fraudulent transactions with all current transaction methods. Credit cards (chargebacks), checks (NSF), cash (counterfeit), etc. As long as that percentage doesn't increase with bitcoins, they'd probably be fine with it.