After troubleshooting this error after a PHP upgrade:
[Wed Jan 12 12:38:49 2011] [error] [client 18.104.22.168] PHP Warning: Cannot modify header information - headers already sent by (output started at /usr/local/www/blog/wp-content/plugins/simple-google-sitemap-xml/simple-google-sitemap-xml.php:156) in /usr/local/www/blog/wp-comments-post.php on line 96
I was able to narrow it down to the following php.ini file variable. When "short_open_tag = Off", the Simple Google Sitemap plugin breaks because the plugin author has mixed <?php and <? tags throughout the file.
The solution was to change <? to <?php wherever it occurred in the file (except for line 65, which is this in my version:
$xmlcontent = '<?xml version="1.0" encoding="UTF-8"?>'."\n";)