Update: The clever fellows over at Netputing bundled the files together with a snazzy install/uninstall button to produce AirPrint Hactivator, which takes care of all the under the hood business for you. The same caveats apply: remember that you are installing obsolete/beta system files, use at your own risk, and if you'd prefer a supported product there are options.
Mac OS X 10.6.5 was released yesterday, and one of the features it was expected to bring to the table was support for AirPrint, a new feature in iOS 4.2 that was supposed to enable wireless printing from an iPad, iPhone, or iPod touch. Unfortunately, whether it's because of technical problems or patent trolling, Apple pulled support for this feature at the last minute. In its current incarnation, AirPrint in iOS 4.2 will now only work with specific printers that come with AirPrint functionality built-in... and you probably don't own one of those.
iOS 4.2 hasn't been officially released yet, but some enterprising developers, Steven Troughton-Smith and Patrick McCarron, have already found a way to re-enable AirPrint functionality in Mac OS X 10.6.5. Apparently, three files from earlier developer builds of 10.6.5 were removed or altered for the final public release, and it's these files that are essential to restoring full AirPrint functionality. Click "Read More" for a walkthrough on how to enable AirPrint.
The usual caveat: This procedure involves a moderate degree of peril, and TUAW cannot help you if you mess up your system. Proceed at your own risk.
What you'll need
This procedure isn't terribly complicated, but it does involve at least one Terminal command. If you're afraid of the Terminal, you probably shouldn't attempt this.
1. You must have Mac OS X 10.6.5 installed.
2. You must have iOS 4.2 on your iDevice. If you don't want to wait for the official release, you can install the 4.2 gold master release right now with a minimum of fuss.
3. You must have the three files from the earlier developer builds of 10.6.5. We probably shouldn't give you the direct link, but if you're savvy enough to follow these directions, you're also probably savvy enough to find the files yourself via a Google search. Once downloaded, the filenames should be urftopdf, apple.convs, and apple.types.
What to do
Once all the required pieces are in place, it's time to fire up the Terminal, because the files you need to move around will be going into folders that aren't normally visible in the Finder (alternatively, if you know how, you can use Terminal commands to move the files into place and skip the "Make invisible files visible" step).
1. Use the following commands to make invisible files visible:
defaults write com.apple.finder AppleShowAllFiles TRUE
Once the Finder comes back up, you'll see an immense clutter of grey-texted files you may have never seen before. That's good.
2. Go to your main directory in the Finder (i.e., click your computer's name in the sidebar of the Finder) and locate a folder with grey text called "usr".
3. Move the following files to the following locations (you'll have to authenticate with your password each time you move files):
4. Once all the files are in place, input the following commands in Terminal to re-hide all those normally invisible files:
defaults write com.apple.finder AppleShowAllFiles FALSE
5. Restart your Mac.
6. Once your Mac is restarted, go into the "Printer and Fax" portion of System Preferences and delete the printer you want to use for AirPrint.
7. Re-add the printer.
8. Print something from your iPad, iPhone, or iPod touch, because now AirPrint should work on your shared printer.
Personally, I've never regarded printing from my iPhone 4 as a critical feature, and I'm betting many other iOS users haven't, either (with the possible exception of iPad users who use iWork a lot). But whether it's because you actually do have the need to print from an iOS device, or you just want to "stick it to the man" and enable functionality we were promised months ago, the above procedure definitely works. I've tested it myself, and I successfully managed to use AirPrint to print an e-mail from my wife with no issues at all. This may be the only time I ever actually use AirPrint, but at least now I know I can use it if I want to.