You should be able to use the D1 with an iPad or iPhone with the right connections. In this kind of setup, you'd just need Apple's USB camera connection kit and a powered USB hub. Here's how you would connect everything together:
iPad/iPhone w/ USB CCK --USB-> Powered hub --USB-> D1 --RCA/Mini-> Speakers/Headphones
The iPad or iPhone should just automatically start playing audio over USB when its connected.