An update is available to WordPress 1.5.1. The text from the release page follows:

In our effort to optimize we made two mistakes in 1.5.1, one related to feeds and one related to trackbacks and pingbacks. We’ve updated the download with 1.5.1.1 which corrects these bugs and a few others.

If you are having trouble with RSS feeds or track/pingbacks, this update is for you.

If you’ve made changes to the existing codebase for your site, the following files were effected, so back them up so you have access to your changes.

wordpress/wp-admin/post.php
wordpress/wp-admin/quicktags.js
wordpress/wp-blog-header.php
wordpress/wp-comments-post.php
wordpress/wp-includes/functions-post.php
wordpress/wp-includes/functions.php
wordpress/wp-includes/pluggable-functions.php
wordpress/wp-includes/template-functions-category.php
wordpress/wp-includes/template-functions-links.php
wordpress/wp-includes/template-functions-post.php
wordpress/wp-includes/version.php
wordpress/wp-includes/wp-db.php
wordpress/xmlrpc.php

Those who use the AuthImage plugin will want to back up wp-comments-post.php.