Lifehacker, always looking for ways to get things done now, has a short list of ways to print from your iOS device without waiting for the wireless printing built in to iOS 4.2. Which will then require support from printer makers. Which will then require you to have a wireless printer.
They point to one of Warner’s favorite devices, the Pogoplug. This device is primarily used to turn any USB drive into a web-accessible networked drive, but the latest firmware allows you to plug in a USB printer, allowing an iOS device to print to it. Another option is the Air Sharing app, which allows printing through a shared printer (though this is limited to Mac and Linux according to the app description). The final option is to use a printer that can print via email. Hop over for more details.