It looks like HP is already creating iOS-compatible printers. Today they announced three new machines: The HP Envy 100 e-All-in-One, HP Officejet Pro 8500A Plus and the HP Photosmart eStation (Above).
They all have the necessary software built-in so that the iPad can send a document for printing easily and efficiently. Rather than having to worry about drivers or major compatibility problems, a printer that you know is AirPrint ready is simply designed to work while on the same network as your other devices.
It seems that little by little, the iPad is becoming a true productivity device.