Then perhaps it was missing the License: tag in the header. In any case, it told me the header was bad when I tried to activate it and would not let me activate it until I added:
Stable tag: 1.1.1
Tested up to: 3.1.1
to the header comment. WordPress says you need:
Plugin Name: Name Of The Plugin
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: A brief description of the Plugin.
Version: The Plugin's Version Number, e.g.: 1.0
Author: Name Of The Plugin Author
Author URI: http://URI_Of_The_Plugin_Author
License: A "Slug" license name e.g. GPL2
at a minimum in the PHP file.