Steve with HiTechnic emailed me some more technical details about the Multiplexor. Very cool stuff here - I hope you're like me and looking forward to getting your hands on some of these new items when they're ready.
Many thanks to Steve for the details provided below. Check out the HiTechnic site here.
The HiTechnic Multiplexor is designed to extend the I/O capabilities of the NXT by providing four additional sensor ports and four additional motor ports. The Multiplexor could be thought of as a mini-NXT, having its own CPU and I/O support hardware. It is powered from an external 9v supply which may be either a battery pack or 9v DC wall transformer.
The four sensor ports are compatible with those in the NXT, supporting digital I/O and able to perform both active and passive RCX compatible analog input as well.
The four motor ports are also fully compatible to the NXT's motor ports. Software within the Multiplexor receives commands from the NXT and performs motor control functions based upon feedback from the motors' built-in rotation sensors, just as the NXT does.
The HiTechnic Multiplexor has a ninth port, the Connect I/O Port, which connects to the one of the NXT's sensor ports. The NXT communicates with the Multiplexor over this connection, transferring packets of sensor data and motor control data.
Built into the Multiplexor is a routing device which permits digital lines to be temporally switched directly through to one of the four sensor ports allowing the NXT to communicate directly with the attached sensor.