According to Simon's YouTube page:
Currently still a prototype, the LEGO Wheelchair can move a 90kg person.
It uses 6 NXT's to drive 12 NXT motors (2 per NXT for maximum power output) which are connected to 12 Rotacaster multi-directional wheels. Using these wheels will later allow for side to side movement(strafing).
There is a master NXT which controls the 6 NXT via turning on/off touch sensors. The joystick is connected to 4 touch sensors to determine direction (forward, backwards, turn left, turn right).
The master NXT then turns 2 motors (left / right side) different directions which turn on / off 12 touch sensors (2 per NXT for backwards / forwards).
Special thanks to LEGO and Rotacaster.com.au for supporting this project.
Simon's wife does the test driving in his videos and the next upgrade will include remote control via any Android device. You can see a step by step photo guide over on his Flickr stream.