I've been testing and upgrading one of my themes for WordPress 3.0. I use a the_post_thumbnail in my theme to display a thumbnail or the default thumbnail if one is not supplied:
<?php if ( has_post_thumbnail() ) the_post_thumbnail( 'thumbnail' ); else echo '<img src="'.get_bloginfo("template_url").'/imgs/thumbnail_default.png" />'; ?>
It seems to grab the thumbnail for the posts with thumbnails, but no longer echo's the else image. What's changed in 3.0? I looked through the Codex and searched the forum, but couldn't find anything.