Arnie, loved seeing that you are no longer building the readme.txt file statically, but are instead relying on WordPress hooks for that. Would love to see the same thing for sitemap.xml if you weren't already planning it. Perhaps use wp-super-cache if its present for caching the file, or just store it in /wp-content/cache/ yourself. Would much prefer to see my document root stay clear of any extra files if at all possible, and looks like you've proved it can be done with the readme.txt file. Keep up the good work on this.
Side note, but you may want to look at http://php.net/version_compare for comparing wordpress versions. For example:
if (version_compare($wp_version, '2.7', '<')) { }
