This is the strangest thing. At home I've got a 24" iMac running 10.5.4, with which I have no problem opening NEF files shot on a Nikon D700. I'm on the road at the moment, and have purchased a used MacBook, which is also running 10.5.4. I've installed all the latest updates, but despite that, I'm unable to open NEF files shot with the same camera using Preview.
Update: I don't see a delete button for posts. I've found out that 10.5.4 doesn't support the D700 NEF format, my memories of using Preview must be with my previous camera (D200). Oooops...
Martin.