If you want to go for the 7 years clinical scholars track at some programs then it is a tremendous asset, otherwise a PhD is not as big a deal as other fields. One of my best friends matched into a 7-year Oto program this year and he said a lot of faculty didn't seem to appreciate that he had a PhD while other applicants did token summer research. This was of course an issue w/ the standard 5 year programs.
If you want to do research, do it in your residency and don't do a PhD in medical school. It doesn't help out an application that much, and definitely not worth the time involved. No to mention you'll be 7+ years past your PhD training by the time you complete residency, much less a fellowship, should you choose one.
Now if you already have a PhD - my experience was that when interviewing, I felt that they were encouraging another 1-2 years in residency for research - which I was not interested in. So maybe it might help if you want one of the 6-7 year programs. Then again you'd be 40 before you start practicing.