Truly Simple theme update
Sean pointed out that if you don’t use the WP2.3 Related posts plugin, you get a PHP error when viewing single posts. It seems I was not using correctly the PHP function function_exists.
How to update
If you already have the theme in wp-content/themes, open single.php in a text editor and replace
<?php if (function_exists(wp23_related_posts())) {
with
<?php if (function_exists(’wp23_related_posts’)) {
If you just want to use this theme, you can get the updated archive here. The link in the original post also points to this updated version.