An approval system can be accomplished using Blocking tasks and vCenter Orchestrator. Basically Orchestrator would listen for tasks to create or clone vApps, and the tasks would not run until a manager approves. Example implementation here: http://blogs.vmware.com/orchestrator/2011/09/this-month-started-with-showing-a-great-example-of-vco-providing-process-automation-using-active-directory-email-and-webvie.html
vCloud Automation Center will probably handle this as well...
