I just installed your theme on my site. Most of my products include some kind of custom options. In Firefox, there is absolutely no problem. But when tryining to add a product with custom options to cart with IE, it simply doesn´t work.
You can test it here: http://www.filzundwolle.de/merinowolle-bunt.html as in your demo store as well. You can´t put the custom computer in the cart with IE.
I found similar posts here http://www.magentocommerce.com/boards/viewthread/210439/ , but am not really able to set it up for your theme. Hope you can fix this issue.
Thank you very much.