This is more of a design bug in WP, really.
I've got a WP plug-in which needs to use Magpie RSS. Because the plug-ins are processed *before* WP's copy of Magpie is included (lines 2-5, /wp-admin/index.php), I can't test for the existence of Magpie. Because PHP has a flat namespace, I can't include my copy of Magpie without WP blowing up in /wp-includes/rss-functions.php on line 16.
I use a clean, distribution version of Magpie that I'm not willing to change. What to do? Swapping a few lines at the head of /wp-admin/index.php to make sure that libraries are processed before plug-ins would be a big help.