I just updated to WP3.4 and discovered that previously available images are no longer "found" by the system. By this I mean that all the image files remain online and accessible, but WP has lost thumbnail data and subsequently shows the standard media icon on the media library list. Front-facing pages that should have images are now displaying the generic "image missing" icon because there is no URL (I checked the source).
My guess is that somehow the connection between database and image listings have broken, and I"m trying to reverse engineer them to find differences. Hopefully there are others with a similar problem and have worked through it?