How to show the product category for a given product in the product list?

Quick Marketpress question here: How (using php in a template) would I go about showing the category a product is in for each product in the product list? Something similar to the price or description in the meta data.

I've already tried putting this in for where I want the category:
mp_category_list($post->ID, '', ' ', '');
and I've also tried:
mp_category_list($product_id, '', ' ', '');
But all I get is "Uncategorized".

I suspect that it's not understanding that I'm trying to point at a specific product.

Thanks for your help!