I don't think it should be allowed.
A comment from the ini_set page [1]:
"If you set something using php_admin_value in httpd.conf it is then
not possible to be set the value at runtime, even if it's NOT
PHP_INI_SYSTEM. "
So, I guess it must be a bug.
Tijnema
[1] http://www.php.net/manual/en/function.ini-set.php#30424 That's what I read too. I just wondered because many times there are only .htaccess and vhost mentioned.