Cloud desktops might first be realised on phones and tablets, where we have already handed over complete control of the platform. At that point, and with people running multiple devices and migrating hardware in some cases every six months, why not attach everything to a cloud account to keep it available more easily? Indeed, we are already pushing (or being pushed) in that direction.
When all your files are in the cloud, all your apps are web services and your phone or tablet becomes a mere thin window onto the internet with no real capabilities of its own, you've already got a cloud desktop of sorts. If you can lose your device and replace it without losing anything and without having to run any backup or restore operation, you are definitely dealing with a cloud desktop.
Mokalus of Borg
PS - My Android phone is not running a cloud operating system in this sense.
PPS - A lot of the apps, however, are basically websites.