Hi Dan, the problem lies with the Masonry plugin, which is a JavaScript plugin that the gridview automatically uses. This plugin controls the display of items on the grid and adds the position values dynamically as inline CSS rules. At the moment for some reason it only adds position:absolute to the item without specifying the top and left position, therefore all items default to the top left position (0,0). That’s why all products are stacked on top of each other.
I don’t know if this problem is caused by the way Masonry is used by the WPEC gridview plugin or by conflict with some other plugin. I guess Masonry is used automatically by the WPEC gridview plugin because there’s no way to configure it from the admin panel.
I suggest you deactivate all your plugins and only leave WP Ecommerce and the WPEC gridview plugin and check if the page works as expected. If it doesn’t, I still advise you to get in touch with the plugin’s developer, as your pro license entitles you to.