Hey Phil. We actually have a black belt who often wears a white belt under his hakama. I saw him when I first started and then he came back after a two month hiatus. I could of swore he was a black belt, but figured I remembered wrong or saw his hakama ties instead. He just felt like a black belt in his techniques. Turns out I remembered correctly. He is a black belt (3rd dan actually I believe)
Ashley, you might find that a lot of Birnakai yudansha wear white belts. Even some of the shihan. Then again, they're not the kind of white that you'd want for a Tide commercial.
So if yudansha have white belts, some mudansha have hakama, and shihans may or may not *really* be shihan...guess it just comes down to how good one's aikido is.