I ran to the barn, had to change lenses, had to change the battery, and by the time I got downstairs again it was nowhere to be seen. Dang it! Oh, but there it was! Waaaaayyyy up there in the sky, just a little white dot. I shot a bunch of photos, most of which are blurry white specks, but even as crappy as they are, it's enough to see random dark feathers, asymmetrical markings, and a lack of any normal ID marks.
Best guess is that this is a partial-albino red tailed hawk. I sent the pics to a friend whose judgement I trust when it comes to birds of prey. Doing a little more research this evening, I've read that albinism is found in red tails. I really hope to see this bird again, when I happen to have The Beast camera in hand! Whatever it is, it's totally unlike anything I've ever seen.