I'm a 4th yr NSU student, and I met a couple of family residents who were finishing their residencies with the hope of matching into derm programs that "promised" them spots. My good friend was derm all the way (had the rank, boards, research) called up every DO derm program and every one said you must complete at least an AOA intership first, then match. He still did three or four rotations but no one could guanrentee anything. So he matched to one of the handful of officially linked DO ortho programs instead. A girl in our class says she matched derm in Philly, but when called on it had to admit she only matched for a traditional rotating internship with the "promise" that she will get that contract next year. Incidently, she did cores at West Palm and was also made "promises". Maybe she will get derm in Philly, she is an impressive candidate and deserves it. But everything I've ever heard is there are no linked derm programs.
The PDs of the fam med/ or rotating interships use the possibility of matching derm to fill these programs. You can't really blame them, usually these residencies don't fill. I had a PD of a DO intership (at a hospital owned by a MD university) try and get me to commit to his intership with the promise of MD anesthesia (even to the point of contracts for both internship and anesthesia offered, and me being told I would not be ranked at the MD program- I could only sign both DO intership and MD anesthesia contracts, thus not go through the NRMP). The reality is the world of DO residencies is shady. I said no thanks because anesthesia isn't that hard to match, but derm is a different story.
Go into getting a DO derm residencies with eyes wide open and believe no one.