It's not a Mountain Lion (10.8) issue.. it's a Preview issue and has existed for at least 4 or 5 OS versions.
There's little you can do about it other than ask clients to use Adobe Reader or Acrobat to view proofs.
Interesting, though I've never noticed it with previous any previous OS. At least not within the Mail app. But I figured this was an "it is what it is" situation. Thanks-
I'm having the same problem. When I open the Acrobat Files, the color looks fine. When I look at a PDF with spot colors in Preview Mode, in a Window Column View, or as an email attachment, I see SOME of the spot colors showing up as black, but not all of them. It seems to depend on the particular Spot Color.