I just became aware of this yesterday. This problem was reported (by our Ian?) in an Apple Discussions thread from Nov 1.

My conclusion is the same as Ian's. I'm still running Panther so I don't have an opportunity to test this out myself, but my guess is that Apple is not properly decoding the raw images. It is likely that they are getting the white balance values from a fixed offset within the maker notes, and this offset will change when the image is edited (even if nothing new is added).

If this is true, then the OSX preview could also be broken by something as simple as a firmware upgrade to your camera. Hopefully Apple will fix this.