What is the effect of this change on retail sales? Won't this make it possible to effectively re-route a transaction after an exchange of physical goods has already taken place?

Or is there an assumption or recommendation that Bitcoin not be used for transactions that are immediate, such as point-of-sale? I know it's always been recommended to wait for at least one confirmation, but up until now, there's reasonable security in accepting unconfirmed as long as you've got a full blockchain by which to compare the transaction's inputs and outputs.