I imported a bunch of older Nikon D70 RAW files and found that I could not do any RAW adjustments with Aperture. Levels would work, but no white balance, exposure, shadows, etc. Very puzzling and I figured that I must have corrupted the files somehow with Photoshop's RAW editor that I had used previously.
This was not the case, however. Quitting Aperture and restarting solved the problem.