Without wanting to sounds like a knob, i think it depends on your epsitemology. For example, i will be using the first person throughout my PhD, because I'm coming from a constructionist perspective and am therefore critical of any attempt to imply objectivity and distance by using the third person, as i don't think its possible.
But from a positivist perspective, objectivity is the aim of science, so i guess a third person should be more appropriate...?