Same problem here – “Your cart is empty” whenever you try to buy something.
Tried upgrading via the wordpress interface from 1.5.0.2 to 1.5.1.2 and the “Your cart is empty” error started.
Deleted the plugin and tried a fresh 1.5.1.2 install – same error.
Replaced the plugin with the old version and it works again.
Using WordPress 3.4.2 and the RocketTheme Gantry Framework.