Connecting a disconnected device deletes all files from ubuntu one account.
Connect a few devices to your ubuntuone account. Upload some files. Remove a device from the ubuntu one account. Delete the files from the removed devices ubuntu one folder. Reconnect the disconnected device. Your ubuntu one account will have all it's files deleted. This is a bug as far as I am concerned. Connecting a new device does not delete files from your ubuntu one account, so reconnecting a disconnected device should also not delete files from your ubuntu one account.
DistroRelease: Ubuntu 12.04
Uname: Linux 3.2.0-30-generic i686
Date: Tue Dec 11 10:53:00 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
PATH=(custom, no user)
UpgradeStatus: No upgrade log present (probably fresh install)