We have had marketpress installed and running on our site for several months now, having completed a number of sales through the plugin.
As of around 24 hours ago, on the product display page when an "add to cart" button is clicked and the user follows through to the checkout page, the page will always say "you have no products in your checkout". When viewed in firefox the CSS of the page is affected too. I have updated to the latest version and the problem still exists. Obviously, this is quite a critical issue for us so I thank you in advance for responding quickly.