If you make a set of clippers connected to a vacuum cleaner such that the hair is sucked towards the clippers, then use a semi-circular track for the clippers such that they can orbit your head. The track can also be rotated in an arc such that it can 'scan' a hemisphere. Then the clippers need to be able to move in and out towards ones head. Then you just need to scan the hemisphere moving the clippers in and out appropriately for the haircut you want!
OK, it may not give a hairdresser a run for their money - but I think it could do a reasonable, repeatable job.
An alternative would be to use a pair of inclinometers and a compass on the clipper/vacuum and use them to calculate where on ones head they are (assuming it stays roughly at a normal to the surface of the head). Then you move the clippers around your head and it adjusts the distance for the cut length - and maybe warns you if you've missed an area?
I've not built one as my hair seems to be falling out on its own fast enough!