I have built a Lego Mindstorms NXT, 8 Input RCX Touch Sensor Multiplexer for use with the old style RCX Touch Sensors.
NXT - 8 Input RCX Touch Sensor Multiplexer Unit without the Sensors attached
I really don't know what the Lego Designers where thinking when they designed the NXT Touch Sensor!
I have one of Mindsensors' new TouchMux's, but it doesn't work very well with the legacy RCX style Touch Sensors. Mindsensors do make a very good a RCX Sensor multiplexer for NXT, the RXMux which allows you to connects up to 4 RCX style analogue sensors to your NXT. The RXMux Supports the following Sensors:
RCX Touch Sensor
RCX Light Sensor
RCX Rotation sensor
RCX Temperature sensor
As I often use more than 4 Touch Sensors in many of my Robots, I needed to construct my own RCX-TouchMux as there is no commercial equivalent to my Knowledge available.