I use an excellent plugin to display my Last.fm recently listened to tracks. You can find the plugin here. After the upgrade to WP 2.8 I kept getting this error when trying to activate the plugin “The plugin does not have a valid header.” After trying a few different things I ran into this post that actually makes sense. Ultimately, if your plugin is contained within a two folders it will be a problem. For example, plugins\lastfm-records\last.fm\ will create a problem, you need to remove the lastfm-records folder and keep everything within last.fm.