I have a WPMU installation used for years. One MU plugin I use is Ada WPMU Sitewide Feed.
Until WordPress upgrade to 3.0. It has multi-site support internally. So, upgrade WPMU to WP 3.0.1.
The problem just found that Sitewide feed does not work any more.
No content in the feed.
I went to the author site of this plugin.
In the comment part, Brad Williams pointed the solution to fix this problem.
The plugin has one file only. wpmufeed.php
Open this file in whatever text editor.
Find line 250,
global $wpdb, $wpmuBaseTablePrefix;
Add following just after the line.
$wpmuBaseTablePrefix = $wpdb->base_prefix;
Save it and all fixed.
Thanks to Author of this plugin, நாதன் Nathan, and Brad Williams.