I'm gonna say it's a selective desaturation -- I noticed, in the example about three pages back
with the castle, that the building and other architectural details were a lot grayer and flatter than the model and the sky, though everything seemed to be altered.
I stole someone else's example and did some desaturation and curve adjustments, treating the model, the stool and the background as three distrinct areas of work.
I also think it helps to CMYK the photo first, which immediately lessens some of its depth.
I also noticed some "random" bursts of color in the samples, so I went back in and added some contrasty pinks back into the models lips and her watch (going with the art-class theory that if you use a color once, you should pick it up somewhere else.)
Anyway, I'm long-winded today, but that's my take.