However, even for unconfirmed transactions, double spending is very easy to detect. You can see the unconfirmed transaction appear in the network right away, and likewise you can immediately see if there were any other (also unconfirmed) transactions being sent from the same address(es). If they exist, and they exceed the total balance prior to the transactions, you simply refuse it. Can be done fully automatically.

In other words, if I were a retailer, I'd simply demand that Bitcoin payments are made from addresses that do not have any pending outgoing unconfirmed transactions that exceed the remaining balance required for their payment to me.

Instant payment FTW.