With Apple's pulling AirPrint of Mac OS 10.6.5, an opportunity has opened up for developers to one-up Apple with 3rd-party tools that can share printers over Wi-Fi for iOS 4.2 devices. TUAW have discovered "Printopia and FingerPrint," -- both products essentially allow printers that don't support AirPrint natively to be made available to iOS 4.2 device, by modifying their Bonjour service broadcast to include an "AirPrint enabled marker."
iOS 4.2 devices can then detect and print to most printers, but not all, mainly based on host-driver requirements.
The $9.95 Printopia, takes things a step further, by sharing non-Bonjour-enabled, local USB-connected printers in the same fashion. Support for printers that are password protected and that can duplex print is also included. It also provides printing to PDF files on your Mac and direct-to-Dropbox file sharing, straight from the print dialog on iOS 4.2.