this is a great idea and a nice plugin.
1. could you please add some code into the plugin, that it would work well even with the relative paths? Say that I have the image as /images/2011/05/apple.png. Then curl_get_file_contents() or file_get_contents() fails to fetch it in apt_generate_post_thumb(). Just adding site domain name in front of $imageUrl would fix the issue.
Without this, it barely works for us, as we convert sites from other blog system a lot and they often use relative paths.
2. Also, it would be nice, if it worked even on save_post, not only on publish_post.