As delivered, AirPrint will work to send print jobs from select apps on your iPhone or iPad to a limited number of HP printers that support the ePrint standard. ePrint gives your printer a unique email address, and you or your application can email PDF files to that address so the printer can spit them out for you. That's different from how AirPrint works under the hood; it leverages Apple's mDNS stack (originally called Rendezvous and now known as Bonjour) to locate and spool to printers on the local network.
While only three HP devices were qualified for AirPrint when the 4.2 update was announced earlier this year, that roster has expanded considerably -- HP now lists seven compatible printers, five that can be made compatible if updated to current firmware, and six more that will be compatible as of December 27 pending an unreleased firmware update. We're currently waiting by the mailbox for our test printer, and when it shows up we'll give it a full workup with AirPrint to see how it does.
So AirPrint is great, if you've got one of those HP printers you're ready to roll. And if you don't? That's where things begin to get interesting.
It didn't take long for inquisitive folk to figure out what had gone missing between the 10.6.5 builds in terms of AirPrint support -- three small files, identified by Steven Troughton-Smith, Patrick McCarron and others. Replacing those files with the older versions told the CUPS printing subsystem in Mac OS X to advertise the shared printers in the AirPrint-savvy fashion. While it wasn't hard to swap the files, it was even easier to click a button, and that's what the Netputing team did when building the free AirPrint Hacktivator.
If swapping the files seems like overkill, a German blogger took note of Ryan Finnie's approach to enabling AirPrint sharing from Linux and built an AppleScript to make the necessary configuration change (one line!) in CUPS. All of these approaches may work for you, but your mileage may vary; please be aware of what you're doing and be prepared to deal with any quirks or instability. (Remember, to go forward we must have good backups.)
On the Windows side, it was the beta versions of iTunes 10.1 that included (and then dropped) the AirPrint support. Similar spelunking by some of the denizens of the MacRumors forums found the necessary older files and gave instructions for the add-back; Jaxov summarizes them. I haven't tested this approach, but the same caveats apply: make sure you're prepared to deal with any instability or issues caused by tweaking your system in this fashion.
Given the opportunity to provide help to iOS users and make a bit of money at the same time, two commercial products are available to take care of AirPrint enabling for you, at least on the Mac: Printopia and FingerPrint. Both are less than $10 and both include a 7-day demo; Printopia also includes the ability to print to PDFs on your Mac, either into your Documents or Dropbox folder for easy access/sharing. I used the Printopia demo to test my printers, and I'm probably going to buy it -- the print-to-PDF feature alone is worth having, and the time I'll save versus setting it up myself is easily worth $10.
Once you've picked an approach and set yourself up, how well does it actually work? The answer is: Pretty darned well. Here's the process:
First, share the printers you want to play with. Without Printopia, this means heading into the Sharing pane of System Preferences and turning on Printer Sharing, then checking your shared printers. (If you're on a work network or school network, check with your IT folk first.)
If you're using Printopia or FingerPrint, you can skip the printer sharing step. Instead, go directly to those apps and check off the printers you want available for your iPad. The screenshot below is from Printopia, and you can see that it's straightforward to check off the printers.
Assuming that your Mac and your iPad or iPhone are on the same WiFi network, you're ready to print. Finding print-enabled apps on your device is a bit hit and miss at this point (HP's list is helpful but certainly not comprehensive), but you can certainly make a lot of headway with Safari, Mail, Evernote and Instapaper. (Print support in Instapaper is a bit ironic, but handy.)
In your app of choice, tap the 'actions' button -- it's the one that features a rightward-pointing arrow. Choose 'Print' from the list of available options; if you don't see Print, either the app doesn't support printing or you aren't using the most current version. That "Check for Updates" button in iTunes is going to get a workout over the next few weeks.
The Print pane lets you set a few basic options, starting with selecting a printer. In the screenshot above, you can see that I have the 'Send to Dropbox' virtual printer selected, which shoots a PDF into my Dropbox in a flash. Tap the Printer button to select from the list of available printers.
Tap your printer to select it, then hit the left-pointing arrow to go back to Printer Options. If you want to select a page range, that's easy enough: tap Range to adjust.
Spin the page wheels to select your page range.
Then go back to the main pane, click Print, and off you go!
Let us know how your iPad printing experience works out over the next few days, here in the comments or on our Facebook page.