This is the case with the new post_class() function which adds in class names for the post box. The Codex puts it like this:

The post_class() outputs the class=”whatever” piece for that div. This includes several different classes of value: post, hentry (for hAtom microformat pages), category-X (where X is the slug of every category the post is in), and tag-X (similar, but with tags). It also adds “sticky” for posts marked as sticky posts. These make it easy to style different parts of the theme in different ways.

Adding it the loop in the right place like this:

<div id=”post-<?php the_ID(); ?>” <?php post_class(); ?>>

This causes problems when you have a theme that you want to be backwards-compatible with prior versions of WordPress. So you need to change it to this:

<?php if (function_exists(“post_class”)) { ?>
  <div id=”post-<?php the_ID(); ?>” <?php post_class(); ?>>
<?php } else { ?>
  <div id=”post-<?php the_ID(); ?>”>
<?php } ?>

Now it works with previous WordPress versions!

For now here is a list of the themes that have been fully updated. The links point to the release page with subsequent links showing the theme in action:

• Big Red – current version 1.0.2 – view
• OpenAir – current version 1.1.6 – view
• Curve – current version at 1.0.5 – view
• Digitalis – current version at 1.0.5 – view
• Future Day – current version at 1.0.3 – view
• News Print v2.0 – current version at 2.1.3
• News Print v3.0 – current version at 3.0.3
• Reclamation – current version at 1.1.3 – view
• Shocking – view

All of these themes are backwards compatible with WordPress 2.6, and presumably with earlier versions too (although that has not been exhaustively checked).

These themes have all had the threaded comments added in and the post_class() addition to the post div.