Gallery, once you've manually compiled/installed dcraw, seems to display NEF images on the fly without any fuss. Currently it's lacking the display of any metadata for such images however. But that's not of huge concern for my current needs.

Gallery also lacks any ability to read xmp sidecar files (for updated metadata not embedded in an image), but again, this interface is primarily just a way to occasionally browse into my backups and a way to work around Dreamhost policy restrictions.