I found these at a local grocer and so I bought a few and tried them. They are GREAT! On the outside they look like regular navel oranges, but the flesh inside is pale pink. They are very sweet, juicy, and are low in acid. Though they look like grapefruit, they do not in any way taste like grapefruit. One I ate straight, the other I segmented and had in a salad with fresh roasted beets and greens. Superb both ways.
For those in SF: I found these at Real Food on 24th Street. Organically grown, of course.