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.

 

~david