(Using 10.3.2 and latest version of Backup)
Upon starting Backup, I get this error message after it attempts to connect to the server:
Backup requires a .Mac membership.
iDisk did not recognize your user name and password. Please make sure your user name and password are correct in Internet preferences or sign up for a .Mac account.
Funny, I can use iSync just fine. My iDisk information shows up fine in the System Prefs and I can sync it by dropping files in it.
Could this be related to a closed port? My school has overzealously closed off a lot of ports in response to MyDoom (can't check e-mail, use AIM, connect to LimeWire, etc.) recently.