With the release of WordPress 2.7 Coltrane came the introduction of a new template tag called post_class(). Interestingly I have been using a custom template tag by that exact name for a long time in my themes including the free Typograph. That means when users of Typograph upgrade to WordPress 2.7 Coltrane a conflict occurs between the WordPress functions.php file and the Typograph functions.php file because both define the same template tag.
To solve this issue I have released a new version of the Typograph theme customized for WordPress 2.7 Coltrane. It has been submitted to the WordPress Theme Directory pending approval. In the meantime you can download it directly from my server: