I just updated Thematic from 0.9.5.1 to the latest version 0.9.7.7. My custom CSS classes based on dyanamic dates stopped working. I was able to replicate this on my development server with a fresh WP install and a tryout of the two versions of Thematic.
As an example:
<div id="post-1" class="hentry p1 post publish author-admin category-uncategorized untagged comments-open pings-open y2011 m07 d15 h10 slug-hello-world">
<div id="post-1" class="post-1 post type-post status-publish format-standard hentry category-uncategorized">