I've tried to upgrade my php version from 5.2.9 to 5.3.1 and there's an important modification in this version : ereg functions are now deprecated. Some wordpress' functions doesn't work : ftp update for exemple.
You should replace all ereg functions by the preg equivalent.
I've listed the file where the ereg functions are used :
wp-admin/includes/class-ftp-pure.php wp-admin/includes/class-ftp.php wp-admin/includes/class-pclzip.php wp-admin/includes/class-ftp-sockets.php wp-includes/functions.wp-scripts.php wp-includes/class-phpmailer.php wp-includes/class-pop3.php wp-includes/gettext.php wp-includes/rewrite.php wp-includes/functions.wp-styles.php