Car rental company SIXT ran an interesting ad campaign a couple of years back where they advertised special offers to airport travellers by using the text in wi-fi network names.
There's no reason why any bricks-and-mortar store couldn't use a similar approach. You could broadcast an offer, or some other call to action which would lead to people connecting to your network. Once people are connected you could direct all traffic to a locally hosted webserver or proxy which could then offer a landing page, access to your website, or if you're feeling particularly generous (and have faith in the common carrier defence), free access to the internet. You could even marry up the free internet access with a bit of Ad Jacking if you fancied.
If there were enough people interested in doing this, it wouldn't be too difficult to come up with some kind of simple hardware appliance to take care of most of the hard work. Just plug it in the wall and wait for the customers to beat a path to your till.
So why aren't people doing this? Well possibly it's not that great a channel since most people don't go round scanning for free wifi. I usually only do it if I'm abroad to avoid being gouged on data roaming, or when Orange's 3G network is proving elusive (which for me appears to be around 80% of the time). So maybe you might pick up some tourists and the odd wardriver.
Also the strength of your wifi signal will be limited so it's probably only really going to attract people who are practically on your doorstep anyway. However if this were to be used in shopping malls or any other large private space (where multiple wifi transmitters were an option) then it could possibly be effective.