NC and ND are clearly incompatible with GNU/FDL. CC-BY-SA on the other hand is incompatible on a technical level only. That is, the rules specify that one cannot cross-license, but the spirit of both licenses is equal. Allowing CC-BY-SA is only a small step from requiring everything to be GNU/FDL. Allowing NC or ND is a much larger step, and we could not reasonably consider an article as a whole to be GFDL if there are such images in it.
Well, true (although GFDL has that annoying requirement to print/include the license alongside the product, which is why I despise it for images), but I guess that raises the more central question of: what is the big deal about WP being GFDL? My understanding is that GFDL was chosen as the time partly beause it was the most well-known copyleft license suitable for text. If we had that time again, would we still choose GFDL? How about CC-BY-SA? How about CC-BY-NC? What is the killer argument that Wikipedia should be allowed to be reused in a commercial setting? (I obviously don't consider the DVD argument to be a killer argument.)
Brianna