According from a report from Macstories, it seems like Apple has discontinued the support for printing from iOS devices to shared printers connected with Macs and Windows computers.
Macstories has reported that:
We have been told from a few Mac developers that a few days ago Apple removed all the references to printing via OS X 10.6.5 and PCs both from Readme files and other online documentation posted in the iOS developer center. It seems like all that’s now mentioned in the release notes are the aforementioned HP networked printers, as if the shared printing option never existed.
If you believe the developer who contacted Apple support regarding this, it seems that indeed company has cancelled AirPrint through shared printers. It still is not clear whether they plan to bring back the service sometime in future or not.
Here is the reply from the Apple Support when contacted:
Support for AirPrint on Windows and Mac has been cancelled. We will be in contact with you if another opportunity arises in the future.
Mac OS X 10.6.5 and iTunes 10.1 which was all set to bring the AirPrint support to Macs and Windows PCs and also the iOS 4.2 are still not out, so we will keep our guesses to ourselves only.