The new release fixes over 400 bugs, including the exploitable $GLOBALS overwrite, several open_basedir issues and the phpinfo() XSS vulnerability. As usual, the PHP team strongly advices all users to upgrade to 5.1 as soon as possible.
Furthermore, PHP 5.1 comes with array type hinting, the PDO extension enabled by default, PCRE 6.2, PEAR 1.4.5 and performance improvements in the Zend Engine II.
PHP 5.1 ChangeLog
Upgrade notes for PHP 5.1