Haha a little bit off topic here isn't it?
About the "GTX260b" being a weakened GTX280. Of course, that's exactly what it is. That's exactly what GTX260 is.
But about being it a way to sell the GTX200 stock, I don't think it's only because of that. I have said thi before, that IMO Nvidia when they design their chips their goal is to make the chip so that the second card can be the same one with one cluster dissabled. But in order for this you need good yields, if you don't have enough of them you have to dissable one more. Yields is the one thing you can improve a lot over the time, so possibly right now dissabling one cluster is enough to assure a high yield rate.