The only way grocery stores could get on the network would be to have some sort of "MYBITCOIN"-like intermediary that does all the transaction processing, most of which would have to happen off the block chain.
What's to stop a service like mybitcoin from transferring between the wallets it maintains with every supermarket purchase?
The way I see it, this is absolutely what would need to happen.
And further, if my merchant account were at MYBITCOIN and your bitcoins were at YOURBITCOIN, chances are good that there'd be an API call between them to "make" the funds transfer, which wouldn't actually transfer on the block chain, but would rather just be added to an IOU tally between the two bitcoin "banks" that was offset by funds travelling the opposite direction (MYBITCOIN to YOURBITCOIN), which got auto-settled in a single bulk transaction via the block chain at the end of the day.