I was trying to achieve this as well, and was sucessful with the approach of looping through the extra fields array. See the thread
http://forum.x-cart.com/showthread.php?t=63314 where I wanted to show extra field data on the invoice (to help us in pulling products to fill the order. The same looping approach worked on the product list pages.
Totaltec...if your suggestion of {$product.extra_fields[0].field_value} works (I assume the [0] is replaced by the index for the extra field value you want to display) then it seems it would be more efficient than looping through ALL the extra fields for each product being displayed while browsing a catagory or when viewing search results. What do you think?