If you have a USB printer that is already set up on your Mac, and try to use it as an Airport wireless printer, you may find that it doesn't work. The printer may show up in 'Printers' in the Airport Utility, but it just won't print. The solution is simple.
Go to the System Preferences » Print & Fax preference pane, and click on the printer in question. On the right, you'll see that the 'Location' is your machine name.
Thats's the problem. The printer is no longer connected to your Mac, so the printer can't be found. Unfortunately, there's no way to reset the location of the printer.
Delete the printer, then click the '+' underneath the printer list to re-add it. When you click the 'Default' tab, you will now see your printer name, and 'Kind' will say 'Bonjour' -- in other words, it's now recognized as a network printer, instead of being directly attached.
Click on the printer in the list, and the information below will fill in, including the 'Location' now being your Airport's name. Click 'Add' and it will work.
This is never stated in quite this way in Apple's setup or troubleshooting documentation. They do tell you how to reset the printing system entirely, but that wipes out all your printers. You don't need to do that, you only need to reset the printer you want to use on your Airport.
[crarko adds: I haven't tested this one. Although it's certainly possible to get a printer working connected to an Airport base, I've never been all that enamored of the idea. Partly because, in order to get maximum range, I often place my Airport in an elevated position in the room. That makes it pretty hard to connect a printer.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110205095724661