Here's a Wired article by security guru Bruce Schneier entitled Steal This Wi-Fi

I can count five open wireless networks in coffee shops within a mile of my house, and any potential spammer is far more likely to sit in a warm room with a cup of coffee and a scone than in a cold car outside my house. And yes, if someone did commit a crime using my network the police might visit, but what better defense is there than the fact that I have an open wireless network? If I enabled wireless security on my network and someone hacked it, I would have a far harder time proving my innocence.

My thoughts on this are twofold if I were to do this (and he has some good arguments on it): 1 - I'd want a 'no wireless' button so that anyone using my wireless is cut off when I'm playing games... nothing sucks like having a high ping for no reason all of a sudden and 2 - I'd probably want to DMZ the wireless so it is separated from the rest of my internal network. Having open wireless is very nice.... it annoys me if I'm wandering into some random building in downtown and want to check my email from my iPod and all the networks are encrypted and not using 'admin and password'. Of course, out in the boonies where I live I doubt there'd be a whole lot of "walk in" traffic as it were :)