If the retailer can make it economically unprofitable to try to double spend then there won't be a problem. i.e., if the attacker/thief has to buy 19 hamburgers before the 20th is "free" from a successful double spend, then an attacker probably isn't going to try. But the merchant isn't looking to figure out a way bitcoin will work, the merchant is looking only to sell hamburgers, and if a payment network gives hassle and risk it is not likely to be embraced.
Sorry to bump up this thread but I think it is relevant given that namecheap has become the first merchant to accept bitcoin payments without waiting for confirmations. So looks like Stephen was at least half right above.
Stephen, if you are still subscribed to this thread I'd like to know what you think of the recent namecheap move? Do you think other merchants will follow suit?