I am just joining the crowd now. Since upgrading to WP3.3 and thereafter to 3.3.1 ( in German), I am facing constant problems.
The files are uploaded to the correct upload directory, but no thumbnails created and no 'guid', they are attached to posts but no 'post_mime_type' in wp_posts and no post meta values inserted under this attachment_id. I've read about a bug here : http://inoplugs.com/updates/wp-backgrounds-premium-1-5-released/
What bug is this? It sounds pretty much like my problem.
Please try to avoid telling me to read through the master list and de-activate plugins! I don't know how many times I have done it! No success!