But today the market is changing. To understand the changes and what all this means I recommend to read at least these 2 blogs:
VMWARE BUYS NICIRA: A HYPERVISOR VENDOR WOKE UP
VMware’s Acquisition of Nicira – VMware confirming the hypervisor is dead
The changes the blogs describe are already happening. As a practical example is a Rackspace Cloud Network product, a hybrid network that was created to leverage the potential in software defined network that data center provider can benefit from.
Further reading & references