I having the same problem: 95% of my featured images are gone. They are still there on the file system. It appears that WordPress sees the attached featured image in the media library for that post as a 0px by 0px image.
I also have a problem similar to @nvtyson.
In the past, I did have to generate these thumbnails, and I used Regenerate Thumbnails plugin. I did this, because many of my posts do not have the images "attached".
During an export and import from one server to another, many of the images became detached. So, I wonder if that is part of the problem.
(side-note: I'd love to know how to BULK re-attach images to posts that already have the images inside them.) More pressing, is how to fix all these featured images without have to manually go through. I have over 170 custom post types that need it and then probably another 300 blog posts missing featured items.
This problem is related to wp3.4. I do not have this problem for the same website using wp 3.3