This is a common occurrence with CMYK PDFs (and other CMYK documents, on occasion) when viewed with tools that are oblivious to CMYK color spaces and try to interpret the image information as RGB. Even Bridge has this issue with thumbnails of CMYK PDFs. It's nothing to be concerned about. Acrobat and Photoshop understand CMYK information and know how to interpret it for an RGB output device (the monitor), so you see it correctly.
As a note, Preview is a notoriously unreliable way to view a press-ready PDF. It's not just the CMYK-to-Screen that's flaky. Bits of the PDF, like certain characters in a font or some images, may not render at all, even though they are there and will print fine.