By Jon Brodkin (Network World)
Server virtualization projects are driven by a desire for consolidation, yet the uncontrolled proliferation of virtual machines can result in just the opposite.
Think server sprawl is bad now? Just wait till you experience virtual server sprawl. When users can clone a virtual machine with the click of a mouse, or save versions of applications and operating systems for later use, you're asking for trouble if IT doesn't maintain tight control...

http://www.infoworld.com/article/08/05/01/Virtual-server-sprawl-highlights-security-concerns_1.html