I just pasted the MW mode into .emacs for emacs 23.4.1 from ftp.gnu.org, and it did not load correctly when I tried to open MediaWiki's index.php file. I don't really know lisp, so I can't really debug what happened.
Topic on Manual talk:Coding conventions
I made a couple fixes that get it working for me (Emacs 23.4.1). They are pending review.
This post was posted by Mattflaschen, but signed as Superm401.