As to your example: doing a double spend attack on a $250 purchase (that is handled by a competent payment processor) is just not financially sane. So yes, I absolutely believe a store that handles such small amounts will accept 0 confirmations. The tiny risk might be taken by the payment processor or the store, that remains to be seen.
Why is it not financially sane to do a double spend attack? If merchant is taking 0 confirmations, then there's probably like a 50% chance that my double spend will work. At checkout my evil wallet software simultaneously sends the bitcoins to the merchant address and does a separate transaction sending them to me. These are broadcast to the bitcoin network at the same time. One of these transactions will get added inside the block chain.
From
https://en.bitcoin.it/wiki/Double-spendingTraders and merchants who accept a payment immediately on seeing "0/unconfirmed" are exposed to a double-spend occurring if there was a fraudulent attempt that successfully communicated one transaction to the merchant yet communicated a different transaction that spends the same coin that was first to eventually make it into the block chain.