I've been playing with the HiTechnic Acceleration Sensor. Here you'll see a small program (a modification of a test program HiTechnic created) that outputs the values of the X, Y, and Z axes.
As the Surveyor rolls up and over objects, I can see the values change. Now what I need to do is translate those values into useful information that can help me determine if the robot is at a dangerous angle (possibly ready to tip over on its side). In Surveyor, I originally used the Acceleration Sensor to determine the up/down position of the camera as well as to center the camera parallel to the floor. (Brian Davis used the sensor with his LNE robot to determine if it was "upside down.")
Surveyor info: http://us.mindstorms.lego.com/nxtlog/projectdisplay.aspx?id=2382bc2f-0e69-40f6-a456-5666b615a97d
I'm working on a special little bot now that will use the Acceleration Sensor to keep an object it is holding in a very specific orientation. More on this soon...
You can find more info on the Acceleration Sensor here. HiTechnic will also soon make the NXT-G block available for download soon, so keep your eyes open.