Exactly the idea of upselling, right at the counter, except it's more custom because you make offers based on what their holding
I've got a first pass done on my site which you can view here:
http://www.mixtapemp3.com/customer/cart.php
Add product(s) and add to cart, then proceed to checkout. After logging in you'll get the recommends (I wanted it after login so I have a commitment from them).
It looks through the products in their cart, and looks through all the order data for those products, pulling our products that were purchased at the same time as the products in their cart. It then randomly picks a set number of items (in my case 4) to return. They can click "show me more" to get another random set.
After adding to the cart, the mini cart in the top right updates, but they stay on the same page with a message saying the product has been added.