See this forum post for a thorough walkthrough:
http://psubuntu.com/forums/viewtopic.php?f=4&t=5∞

Making the sixaxis work in PSUbuntu in USB mode

SixAxis will 'just work' when plugged into the PS3 via a USB cable. You can use joystick aware applications at this point. Joystick nodes will be created at /dev/input/jsX, with X being the joystick number, starting from zero, ie. /dev/input/js0. Be sure to press the PS button after plugging in the SixAxis, otherwise no signals will be sent, even if the lights are flashing.
If you get erratic or no response from programs whilst trying to use the SixAxis, be sure you check your settings in that application, and then try using the jscalibrator program which can be installed in synaptic, or by typing the following in a console:
sudo apt-get install jscalibrator
then do:
jscalibrator /dev/input/js0
Also, you can test that input is being sent to your console by using jstest which will show the 'raw' input being sent to the console:
jstest /dev/input/js0

More advanced topics

You can also use the bluetooth features of the PS3 and SixAxis to use your controller wirelessly. This requires a bit of work, but a tutorial is provided here:
http://psubuntu.com/forums/viewtopic.php?f=4&t=37∞
You can use the SixAxis to control your mouse pointer as well. Be sure that your controller is hooked up and working with your PS3 before X is started, or have a way of restarting X (ie. by logging out and in again)
http://psubuntu.com/forums/viewtopic.php?f=4&t=5∞

This has not been tested, but it may be possible to install a windows emulator, such as Wine, and use the Sixaxis drivers for windows. The driver and instructions on how to set it up are available Here∞, but you will have to register in order to download it.