I agree with the following posts. From what I have seen at my institution, the language tends to follow as such:
In the context of a clinical department, there are fellows, and some chose to have protected research time extending from 1 year to several years. Some of these individuals are MD and some are MD/PhDs. For those that spend several years, they work on getting K08's and breaking off being independment PI's. In the lab, these individual are typically called a fellow; however, the term post-doc is occassional used. My research program currently has one MD and one MD/PhD transitioning from fellows to associate to tenure track positions with their own labs. The terminology gets admittedly fuzzy as you progress down that road. Both of them now have their own post-docs, techs, and undergrads.
Post-doc is generally a term reserved for PhDs.
I have yet to see an MD-PhD do a research post-doc without some level of clinical responisbility (aka not fellows). Realize this is just what I have seen and I understand that does occur.
I don't know if the above helps answer your question, but this is just what I have directly observed as an MSTP student. As a result, I think as a MD/PhD the optimal track is: Residency --> Fellow --> Faculty. No post-doc added on but realizing that the fellowship is extended to include the neccessary clinical training plus enough protected research time to establish yourself as an independent scientist such that you can develop your lab. In otherwords, the fellowship is both a clinical fellowship and "post-doc" in one.