Thought I might post this for anyone getting this error message for certain plugins after upgrading to WordPress 2.8:
“The plugin does not have a valid header.”
I had the same problem with two plugins I was using - wbQuiz and Random Posts Widget.
By moving the main php file out of a subfolder and placing it in the main plugin root folder the problems sorted itself out and the plugins worked.
E.g. For the wbQuiz plugin
The wb_quiz.php and wb_quiz_question.php files were located within the "plugin" folder off the main root folder i.e.
By moving these files out of the plugins folder and into the root folder (along with any other tests the user may have created and placed in the plugins folder) the problems in WordPress 2.8 is rectified
i.e it should be structured like this
I found the same thing with the Related Posts Widget plugin. The related-posts.php file was originally located in a subfolder off the main plugin root folder i.e.
By moving the php file up one level to:
the plugin worked perfectly and no error messages.
Not sure if this will work for everyone else who is getting the “The plugin does not have a valid header.” message but worth a go.