Apple needs to bake AirPrint into the airport and time capsules.
Not likely to happen, unless the Airport hands it off to a computer somewhere to process. The Airport would have to run enough of the OS X (or some other OS) code to also run print drivers. And then do a translation similar to what Printopia does to take the content from the iPad.
Over the years, printer manufacturers removed more and more logic from the printer hardware and added it to the drivers. The downside is that even if Apple added support to print off the iPad to some older standard, it still wouldn't print to many printers out there.