Basically Wifi Protected Setup (WPS) which is the push-button or PIN based method of setting up WPA/WPA2 security quickly can be compromised (via the PIN method) within 10 hours. I've had mine disabled but a lot of people will have this on by default, even if they don't use it.

http://www.kb.cert.org/vuls/id/723755

http://sviehb.wordpress.com/2011/12/27/wi-fi-protected-setup-pin-brute-force-vulnerability/

Edit: fixed first link.