[Theme: Twenty Eleven] Menu bar loses style when in a frame viewed in IE9

When a Twenty Eleven site is placed in a frame (either frameset frame or iframe), the menu loses its css style and becomes a simple list of links down the left side. This only happens in IE9 and looks ok in Chrome and Firefox.

For example, this page places the Twenty Eleven preview in a frame.

Any ideas?

http://wordpress.org/extend/themes/twentyeleven/