It's not clear to me that your example actually motivates a new technology. If these VMs all have IP addresses, then they can all talk to each other already -- no special subnet is needed. And unless this virtual subnet is performing VPN-style encryption, this virtual subnet does not provide any additional security or data isolation on top of what TCP/IP already provide (ie. you can only read data addressed to you unless you control the physical hardware and put it in promiscuous mode).

I'm not contradicting the idea that this technology is useful (obviously VMWare's acquisition speaks for itself), but I would like to see a motivating example that can't be accommodated with plain TCP/IP.