Ha ha!

Well, my problem was unique to me. Total operator error.

I had edited my product display page so that I had the Add To Cart button at both the top and bottom, and hen I added the product attributes, I also added them to both the top and bottom.

So it was defaulting the radio button correctly--it was doing it in the second set of radio buttons because that's the last one it saw with that id.

I was looking at the top set of radio buttons and that one wasn't getting set.

I guess the moral is, it's ok to have an Add to Cart button in more than one place, but not attributes if you're going to have defaults.