It'll be a neat trick if Apple completely prevents it. The camera kit opens up file storage on an SD card or your camera when plugged in via cable. So you know iPad will allow moving photos and video on and off either. I'll bet they'll let you save iWork documents to SD or USB thumbdrives, mainly to make it easy to transfer them to a PC you can print them from.
I KNOW the iPad doesn't have more than a single proprietary dock port because Steve J is anal about the easthetics, and he can force 3rd party manufacturers to pay him a license fee for anything they want to connect there.