Thanks! WordPress 3.6.1. with minimum plugins. I disabled them one by one until I discovered that it was yours that was blocking the featured images. That said, I have the StyleShop theme from Elegant Themes installed, along with their Elegant Builder and ET Shortcodes; in addition, I have the latest Jetpack, a thumbnail regeneration tool (not currently in use), Slideshow and your plugin, version 1.0. The theme and all plugins are current.
When your plugin is active, if I try to set a Featured image, the media library modal window displays one weird blank “image” at the top of the list, and no matter what image I select from the media library, it never “takes”. As soon as I deactivate your plugin, that mysterious blank image disappears and I can select an image for the Featured Image. This applies both to pages and posts, and applies whether it’s a new post/page or an existing one that I try to update. It also applies if I’m trying to change an existing Featured Image or add a new one to an existing article. Basically any permutation I could think of…
Please let me know if you need any other information.