I'm using a hp ElteBook 8540w with Windows 7 x64 as host and a Windows 7 x64 virtual machine running inside.
I added the webcam to to the usb filter:
carefully terminated Skype ( 1 off the 4 mentioned ways works for me ) in the host to avoid accessing it the webcam.
Activated the usb device in the VM:
When I click the allow button on http://www.testwebcam.com/
The blue led by the webcam goes on and the VM freezes and can't be killed by taskmanager ( cf. here )
From this question I guess, that it is possible in some cases to use the webcam from inside the VM, but I didn't find out how to do it on my machine.
Meanwhile I tried the same with an Ubuntu guest and have exactly the same result.