IPhone – AirPrint in photos app

Its not quite AirPrint, but have you tried the official HP ePrint Home & Biz application?

I have it installed on my iPhone and it picks up and prints to HP Photosmart C4500 really easily. Also, installing this application adds a "open with" option when you are viewing a compatible document (pdf/doc/etc) in the web browser / email application etc. That means you just click "open with" then select Hp ePrint and it opens the document straight into the app ready for printing. you even get a print preview as you would printing from your desktop.

Apple routers generally have no viable software you can customize. Other generic and third party routers often can be flashed and updates. It might be possible to get this working on routers that are able to run pared down versions of Linux (such as DD-WRT or Tomato) in place of their stock firmware. Linux can do AirPrint sharing of standard printers with a combination of CUPS and avahi (Bonjour) following instructions you can find here. Some of the 3rd party router firmwares already have CUPS configured, and I suspect you could get avahi working on them as well, there may be a pre-built version, but you may have to compile yourself.

Bottom line, given a compatible router, you should be able to do it, but it will require a fair bit of hacking and googling. There's no turn-key solution that I'm aware of, and of course this only works with routers that can use 3rd party firmware (there's a pretty good selection, but definitely not all), and Apple base stations definitely won't work unless Apple itself adds the functionality.