HiTechnic will start taking orders for the NXT compass online today. It will begin shipping on Tuesday, October 10. Cost is US$46.00 + shipping costs ($7.50 US and $18.95 International)
From the HiTechnic website, some guidelines/rules to remember:
The HiTechnic compass sensor will only operate correctly in a horizontal plane so you must keep the compass level for it to read correctly. This is very important so remember this when you build it into your robot.
NXT Firmware version 1.03 must be loaded in the NXT for the compass to operate correctly. You can check the firmware version by displaying the NXT Window in the Mindstorms software.
It is highly desirable to mount the compass at least 6 inches (15 cm) away from the motors and 4 inches (10cm) away from the NXT brick itself.
Try to make sure it is firmly mounted, if it bounces around, the readings may bounce around too.