That's the thing though. You may not have to rely on apps to use this feature in the future. Websites, emails, etc can add passes to your passbook.
Sample case would be you signed up for a email program at retailer, retailer emails you a passbook friendly coupon, from the email it's supposed to let you add it to passbook, no app required. Same with websites which is why that one that's going around to test passbook works. It generates the (XML?) that passbook and the iPhone recognizes and send the pass directly to passbook from safari.