With the upgrade to latest firmware 4.2.1, you can now print from the iPad to some selected and approved printers.
However, it will only officially print from printers labeled “AirPrint-compatible,” which most probably most users will not have. There is however a mod to overcome this.
AirPrint Activator is a small application that will enable a Mac OSX 10.6.5 and iTunes 10.1 hidden feature to allow your shared printer to be visible on your iOS 4.2.1 devices. AirPrint allow you to print from new generation of Apple iPhones, iPad and iPod Touch to a printer shared over your personal WiFi network.
Source and instructions to use : http://netputing.com/airprintactivator
Video : How to share a printer with AirPrint Activator. Make sure you select your printer that is listed on your Mac and not ” CUPS PDF” as in this video.
Listing of supported printers , thanks to DragnGT and other forum members at MacRumour.com :