I had this working before :
echo ini_get(\"memory_limit\").\"\\n\"; ini_set(\"memory_limit\",\"256M\"); echo ini_get(\"memory_limit\").\"\\n\";
Works for me, has nothing to do with PHP 5.3. Just like many such options it cannot be overriden via ini_set() when safe_mode is enabled. Check your updated php.ini (and better yet: change the memory_limit there too).
ini_set()
php.ini