Building an External Power Supply for the NXT
Phillipe Hurbaine, aka Philo, another regular Lugnet robotics user has put together a replacement battery box plate which adds the capability of using an external power supply with the retail (non education) version of the NXT.
The result is not quite as pretty as the original battery box. On http://philohome.com/nxtsupply/nxtsupply.htm he has laid out the plans, and there should be enough info to build one, and it is actually fairly simple, terminating with a standard cigar shaped DC socket. Good stuff Philo.