How can I build a wall mounted adjustable height desk?
one simple answer and one complex answer... 

first: is to use twin slot shelving to create a large, adjustable shelf/desk.. where you could quite easily create a light duty desk. I have an image, however the shelf in the picture is too small to be a desk, i have personally used a heavy duty version as my desk (safely holding one desktop pc, CRT screen and printer) 

Second, is to use some sort of linear drive where you have the shelf run up and down some form of runner (or drawer slide)..  this would be something close to the screw jacks that are used to jack up your car at the service station. - probably  a little too complicated for use as a desk.