As mentioned above... their is already an app that allows you to print from your iPhone. There was also an article 2 or 3 days after the Keynote with a quote from an Apple source stating they will be implementing wifi printing across networks as well as file sharing via network drives.
With iWork as a native app, and the ability to hook up to a projector for presentations, I would feel pretty safe betting the iPad will come with the ability to print via Wifi as a native capability.