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.
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: ubuntuone-control-panel-qt 3.0.1-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-30.48-generic 3.2.27
Uname: Linux 3.2.0-30-generic i686
ApportVersion: 2.0.1-0ubuntu13
Architecture: i386
Date: Tue Dec 11 10:53:00 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
PackageArchitecture: all
ProcEnviron:
LANGUAGE=en_GB:en
TERM=xterm
PATH=(custom, no user)
LANG=en_GB.UTF-8
SHELL=/bin/bash
SourcePackage: ubuntuone-control-panel
UpgradeStatus: No upgrade log present (probably fresh install)
The files are deleted because there is a metadata in ~/.local/
Technically "disconnecting" the service from the local machine is a simple removal of the token from the keyring and does not execute any method within Ubuntu One synchronization daemon itself.