Not an app, but a possible solution. Plus, IIRC, you had been considering a NAS:
http://www.synology.com/us/products/features/print_server.php
'Now Print From Your Mobile With AirPrint
For those who have an iPhone /iPad /iPod touch, you will be able to print photos, email, web pages, and documents wirelessly from your handheld by connecting an AirPrint-enabled printer to your DiskStation, as long as all the devices are within the same network. There is no learning curve to even set up the AirPrint server, just plug-and-play, and it is ready to use. You can print from your room, the living room, the kitchen or just about anywhere. It’s not necessary to purchase the latest printer model on the market; the Synology DiskStation enables existing printers to become AirPrint compatible. Check the Synology website for the complete support list. '
List of printers that are supported for this function:
http://www.synology.com/support/faq_show.php?q_id=116&lang=enu#printer