Where are NetworkManager's WiFi settings stored?

If you are using NetworkManager (which, I believe Xubuntu does), it should be in /etc/NetworkManager/nm-system-settings.conf.

There is more information about it here: http://live.gnome.org/NetworkManager/SystemSettings.

On Fedora wifi settings are stored in directory /etc/sysconfig/network-scripts. Each wifi network has two files ifcfg-SSID and keys-SSID