I am running PHP 5.1.1 (and previously 5.1.0) and have had no problems with valid pages that omit the closing tab. However, I did experience a problem with blank pages being returned when there was an error in my local.php file earlier today (specifically one of the queries I was using with the mysql auth needed quotes that I had failed to escape).
I suspect the blank pages are returned because inc/init.php suppresses error output on the include_once statement that processes the local.php file, so if an error occurs while processing the file then a blank page is returned. Would it make more sense to use file_exists to test if the local.php file exists and then include it without error suppression to prevent the mysteriously blank page problem from occurring in this case?
There is a warning about the error-control operator at http://us2.php.net/manual/en/language.operators.errorcontrol.php