But other reviews I've seen, including MacWorld's, seem to imply that printing to a shared printer isn't supported.
I've built printing into my iOS app, so I'm a bit desperate to determine what the status is exactly. It's so frustrating because it works fine in iOS 4.2 beta, and the difference between AirPrint-compatible printers only and supporting any printer is HUGE (obviously).
I'm quite sure that Apple has stated that officially the only printers supported by AirPlay (now) are those few HP models listed in the press release. I've seen a few statements supposedly made by Apple people that always include the words "for now." I suspect what happened is the beta test either revealed that the setup process was "too weird" or they ran into a number of nasty bugs and decided to hold off until they could nail it all down. Whatever the real reason is, though, Apple doesn't seem to be talking about it.