mindsensors.com will soon be releasing its Pneumatic Pressure Sensor for the LEGO Mindstorms NXT!

This new sensor will be useful for those who use LEGO Pneumatics with the NXT as its range will be capable of measuring pressure levels from 0 to 35 PSI or 0 to 240 kg/sqcm.

The sensor will be compatible with NXT-G, RobotC, and NBC.

Update:
Following up on reader questions, we were able to obtain more details from mindsensors.com.  The pneumatic sensor can measure up to 35 psi, it is used as a light sensor (i.e. it is an analog sensor, not digital, see here for a review on the difference) and when used in raw reading, it can provide sensitivity of 20 counts /psi. Please to refer the following plot.