Il semble que c'est Quicktime…
OK... I finally discovered the source of the problem.
VTDecoderXPCService is the sandboxing for video content in Lion. All things that are QuickTime related, audio or video, are sandboxed by this service. It just clicked as to what it is. We have iCam for our iPhones. It allows us to remotely view from the webcams on our machines. So... because iCamSource is a QuickTime-related service, VTDecoderXPCService takes over. So when iCam is operating, it's sucking up 25% of the CPU. Which I'm going to report to the developer. :-)