"It looks like an object element that's pointing to
a non-existent page (http://www.webstandards.org/404/)
should be falling through, but that page is
now returning a response status of 200 (success) instead
of 404. Was it previously correctly returning a 404
response?"
No. Better standards support has been on the IE roadmap for a while. They aren't exactly breaking any speed records doing so, but this is consistent with the direction set by IE7, IMHO, and what MS has been publicly saying for quite a while.
