I've heard other people argue this before, but I'm not totally sure I believe it. I've been curious about this for a while, so I dug around a bit online.
Hard data is hard to come by; although the NRMP has reports dating back to 1986 on their website, dermatology only started to transition over to the match system in 1997 (before that, dermatology had its own "late match" that took place during the spring of your intern year). By the time all the programs had switched over to the match in 1998 (only 14 years ago, not a full 20), the percentage of unmatched US seniors applying to derm was 24.2% (second only to plastic surgery at 25%). For the sake of comparison, the percentage of unmatched US seniors applying to rad onc was just 2.7%, in rads 4.3%, and in ortho 20.7%.
http://www.nrmp.org/data/resultsanddata1998.pdf
Going back further, back in the 70's and 80's American residency programs began accepting significant numbers of FMGs. While it's fairly common to see (now middle aged/nearing retirement) 1st generation physicians from Asia and the Middle East in IM, rads, surgery, etc, I've noticed that they seem to be few and far between in dermatology.
Looking at the data, apparently in 1973, FMGs made up 28% of IM residents, 22% of plastic surgery residents, and only 8% of derm residents. In fact, derm consistently had a significantly lower proportion of FMG residents compared to other specialties for the next 11 years (which was all the data available in that paper ca. 1986).
"Dermatologists in the Year 2000: Will Supply Exceed Demand." Arch Dermatol -- June 1986
Interestingly, a couple of these articles from the 80's were predicting a massive oversupply of dermatologists by the late 90's, and were lamenting the fact that the specialty would no longer have a reputation of attracting the "best and the brightest" if the over-proliferation of new residency programs wasn't stopped. The article above ended with the following thought:
"One striking example of a professional group for which supply increased more rapidly than demand is doctors of philosophy in the humanities. Will dermatologists also be driving cabs in the year 2000?"
Anyways...back to the original topic...