"A virtual network is a network you control that runs on top of another network that you don’t control."
Note that the virtual network, in this definition, allows for a rich realization of what the virtual network is. It could be a complete enterprise network replete with multiple IP subnetworks ... or, it could be a small test network with primaily only layer 2 networking involved among the network elements in the cloud. The challenge to cloud service providers is to allow cloud subscribers the highest flexibility in composing latter's virtual networks consistent with the economics of the former's service business. In fact, for some cloud subscribers, there is a need to keep their entire enterprise network assets in the [public or private or hybrid] cloud, and access their enterprise network in the cloud through Internet connections.
Such flexibility seems to be the goal of a new contingent of vendors that have lined up as part of the Open Networking Foundation. If this foundation's work succeeds, the role of networking devices such as the Cisco CRS-3 will be rewritten.
I look forward to listening to Chris and others in the Cloud Computing & Virtualization SIG: Virtual Networks next week!