I've been using CS5 for awhile now. I've been saving PDF's as digital proofs and submitting them to our rips without any problems. Acrobat has various options as you save out a PDF for various applications (print, web, etc...) Be sure to make sure you are using the correct color spaces and saving the file as the correct type of PDF. Hope this is helpful.
- BC
When I open CMYK PDF's with my preview app, it does appear washed out because it is trying to simulate paper color on an RGB screen. if you save it with an RGB color space, it should look right.