WiFi Joker is a upcoming app in Cydia that will allow you to basically break wifi networks so you can join them with your iPhone. The exact description from the developers site is: WiFi Joker, an application that lets you connect to password- protected WiFi networks on your jailbroken iPhone or iPod Touch.
For now not very much information is available on the home-site. There are some screenshots + and a video (embedded below).
The developer will give a live demonstration tomorrow which you can follow on this page.
Wi-Fi Joker is said to be able to break WEP and WPA networks, although WPA is not yet working. Release date should be by the end of this month.