At home I add things to the iDisk as well but also use the Drop Box on my iMac for docs I want to get to while on the road. The Drop Box service also has an app for the iPhone and syncs across computers as well as shares to friends, which Apples iDisk allows that too.
This is something I'm curious about too. I have MobileMe and plan to store some of my movies on my iDisk. How can I access these remotely from my iPad?