WordPress MU is a hacked version of WordPress that can manage several blogs from a single install. Basically, most of the things (plugins and themes) made for WP work for WPMU (don't take this as a golden rule though)
Today on wp-hackers' mailing list, Donncha posted an interesting message about two fundamental differences between WP and WPMU, regarding plugins and user roles.
The main difference between WP and MU for plugins is where the plugin lives. There is a plugins folder still, but there's also an mu-plugins folder too. Any file in mu-plugins will be included/loaded on every request.
Also, almost every user on an MU site is untrusted, even admins. The only people who should have access to edit css or html in your plugin are site admins. You can check if the current user is one by using the is_site_admin() function, any time after init.
I wasn't aware of this, being relatively WPMU ignorant in general, so I thought I might emphasize this Donnchaism for anyone else in my case :)
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