I haven't tried it, but it seems you could open your iDisk and use Tri-Backup or Data Backup to back your files up to different folders on the iDisk. I don't think Apple's backup utility could handle it and both of the ones I suggested will backup to a server. It would be slow and you would have to manually open and close the iDisk, or I suppose it would be possible to write an AppleScript to handle the opening and closing chores.
iMac 3.06 GHz core 2 Duo OS X 10.5.8
MacBook Air OS X 10.5.8
MacMini core 2 duo OS X 10.5.8