Hiya,
I've only been using these forums for a short while and I've already noticed that 90% of new users don't use the code tags correctly - granted that this is because it's been edited from default. A vast majorty of moderators time must be taken up going through new posts and formatting them correctly - I've noticed 'astions' (username) is constantly changing user posts to the PHP format.
Now, as this is a PHP forum, would it not make sense to have the code tags default to PHP? As far as I see it this is a 30 second task, just do:
str_replace('[ code ]', '[ code=php ]', $post_body_var); // Obviously without the spaces
Just my ten cents worth anyway.