VMware announced its new product NSX. In comparison it should be the same for network virtualization what flagship iESX product was for host virtualization.
This 2 links give a glimpse into the architecture design:
These 2 pictures from the links above give a high level overview what it does and how it should work:
On the VMware page we can find as well as these two videos below that present the virtual network challenge for cloud deployments and shows how to solve it with the help of VMware NSX platform.