This is just a note for anyone who went to upgrade to Wordpress 2.5 and suddenly saw a “call to undefined function” error spring up. You need to go into wp-includes and delete plugin.php

Then grab a fresh copy plugin.php from the installation package and upload that to wp-includes

For some reason, that solves the problem. I don’t know why, but it did for me.

Just a word on the install I HATE the new interface. Literally want to throw this thing against the wall, sorry but its true. 2.5 is an upgrade that actually changes some things so you may want to wait and look more into it before upgrading if you like everything working as is.