I believe the reason this has got so confused is that the earlier betas had a driver/OSX thingy you could install which enabled your printers to show up in AirPrint. However they decided (with no explanation that I know of) to pull this feature for release. What happens, though, is that people who installed that beta driver on OSX still have it and if they are attempting to review 4.2-release on the same machines, they might mistakenly be thinking it has that support with it because the old beta stuff is still installed on their system and so their iPad will happily see their printers and use them.

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.