WordPress 2.9 has added new functionality which allows users to use themes located in non-standard directories. It seems as though a variable that I used now contains a full path instead of a path relative to wp-content. It may require quite a rewrite of the plugin.
Please, don't upgrade to WP 2.9, if you want to use this plugin, until I have fixed this. Sorry for the inconvenience.
http://wordpress.org/extend/plugins/wp-theme-showcase-ext-and-i18n/
