actually ive heard from my dean(former PD) and others that programs really dislike it if you don't mention/they don't what you are going into in your personal letter, also it just looks fishy. i wouldnt write the whole thing as if you were going into IM, i think that's a mistake regardless. i think most people just take their neuro PS and tack on a paragraph at the end regarding how important IM training is for neuro etc etc. and submit that, at least that's what I did(and i know many others who did that), some people just submit their neuro letter as is...
scm