Ok, you get a PDF from a client and use Quicklook to look at it, or Preview or toss it into FCP 6/7 and some of the pretty colors the so-called "Creatives" over there made are now showing up as black.
This is also a problem on some versions of iOS it turns out but this post is about OS X.
Simple solution? Use Adobe's free PDF viewer.
The under-the-hood solution is that some versions of Mac OS and FCP and Preview don't understand things called Spot Colors. In a way Spot Colors are kind of like RGB and CMYK only they're different.
Bottom line is that in order to display the colors correctly update Mac OS to the latest, say Mountain Lion or open the PDF in Illustrator or Photoshop, you may have to add in a white or whatever background then save it out as a TIFF or Jpeg or something. Then dump it into FCP.
From the other side, a better but more complicated solution is to convert the PMS values to CMYK right in Photoshop or Illustrator.
Basically open the PDF in Illustrator and Save Adobe PDF and for Destination select something like "Document CMYK - US Web Coated (SWOP) v2" or something similar. Don't include profiles, may help. I'm not sure.
Another option is to select Edit > Edit Colors > Convert to CMYK then save the PDF.
What works consistently for me is this however: