Re: Simon853's post, here is an extract from the Mull enquiry which describes what the Radalt is doing. It may be helpful in answering flyhelis query also.
The radar altimeter looks at the nearest return it gets from the terrain, so if you are flying along next to a cliff it should return a distance to the cliff rather than a distance to the ground beneath you. There is effectively a cone over which the radar altimeter works which is fixed relative to the aircraft, so the actual piece of terrain which the radar altimeter will indicate the distance from is dependent on the aircraft attitude and the profile of the terrain.
Re. Aesir's glacier point, wasn't there a crash in a whiteout in the last couple of years that was put down to the unreliability of radalt over ice/snow ?