Could someone please tell me why there is now a div tag wrapping my signature HTML?

Well, in the very general sense, it's there because chromatic wrote a patch to put it there, and I approved that patch. It's not likely that the site CSS will ever do anything with it, but it is a hook which individuals could use to change their personal view of the site.

I approved it because it did not make any change to the way people in general see the site, and thus it seemed like a pure win. Had it been a more significant change, prior discussion might have seemed apropos.

As things stand, the change will either be made consistent, or rolled back. At this point it already seems more trouble than it is worth, so I'm inclined to favor the latter, though perhaps someone will make a good case for keeping it.

Update - elided duplicate sentence