If this were built on top of (or in conjunction with) Openstack swift (http://swift.openstack.org), it could be done with a common storage backend. There are already several public cloud storage providers using swift (Rackspace, HP, Softlayer, Internap, KT, and others), and a growing number of private deployments as well (Wikipedia probably being the most recognizable name).

If you'd like to talk more about this, send me an email (address in profile).