TUAW posted a guide about enabling AirPrint for shared printers

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.

  AirPrint: how to make it work for shared printers