A good number of popular themes don't currently work with the overlay. I go the the main page, and when I submit a search query the page reloads rather than the overlay opening.

Themes:

• 2012 http://gibrown.wpsandbox.me/demo/?blog_id=20115252&theme=twentytwelve
• 2011 http://gibrown.wpsandbox.me/demo/?blog_id=20115252&theme=twentyeleven
• 2010 http://gibrown.wpsandbox.me/demo/?blog_id=20115252&theme=twentyten
• shapely http://gibrown.wpsandbox.me/demo/?blog_id=20115252&theme=shapely