When I first saw the change in sidebar behavior, I posted a help request to Crosstalk and it was explained that things were working correctly. Since then, a post has been added to the Product Kinja (which I've linked), but since it's also explained in my post and the thread which includes Lauren's reply, I'm embedding them below.

Between my two paragraphs, Lauren's comment, the thread which sprang from it and her post to the product Kinja, I'd say the sidebar change could be pretty well explained.

(I've updated the original post to say that this apparently also applies to the ad-supported blogs)