I am trying to figure out how the prior admins managed to get some guest network adapters into an "invalid device backing" state. According to a support article (http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2008092) it's due to removing a dvswitch in an unsupported way. Anhy thoughts on what that "unsupported way" might be? All the admins deny being anything less than perfect so without some sort of "gotcha" from a log I can't call them on it.
SnapOverflow
It looks like one of the ways you can accomplish this is to set permissions on the network settings such that no one but the VMware admins can see any networking. In that case all VMs with networks you can't see show up as invalid backing. This makes it interesting for those same users to create guest VMs on the proper network (but that's a whole different issue).