Brad Hedlund's explanation "What is Network Virtualization"
Bred Hedlund is very well known netwoking expert. Now he works for VMware | Nicira participating on VMware NSX product which should be next network virtualisation platform (aka network hypervisor). He is ex-CISCO and ex-DELL | Force10 so there is big probability he fully understand what is going on.
It is obvious that "dynamic service insertion" is the most important thing in SDN. OpenFlow and CISCO vPath is trying to do it but each in different way. Same goal but with different approach. What is better? Who knows? The future and real experience will show us what is better. Jason Edelman's blog post very nicely and clearly compares both approaches.
CISCO as long term networking leader and pioneer has of course its own vision of SDN. Nexus 1000V and Virtual Network Overlays Play for CISCO Pivotal Role in Software Defined Networks. Very nice explanation of CISCO approach is available at