There are 4000+ items in the coverartarchive collection that do not have images in them. They appear to all be from the last 2 weeks, and there are 100+ from today alone, so this appears to be an ongoing problem. You can find the items here, with most recently added listed first:
http://archive.org/search.php?query=collection%3Acoverartarchive%20AND%20NOT%20format%3AJPEG&sort=-publicdate