Re 3.5 media problems, I just went through this with several sites. As it turns out, when I deactivated “ALL” plugins and then went back and activated one by one (and then did update) one by one, everything started working fine.
NOTE: Once all plugins were deactivated, the media uploader works fine. So test EACH AND EVERY plugin to see where the culprit is.
Problem solved (I love the new media uploader).
Hope this helps someone.