It's more complicated - Skype doesn't use VoIP protocol, it use propertiary, closed version of something like p2p network and voip bundled together. At anytime, there may be (and is) shitload of data flowing through your machine, tunneled by skype. Considering many direct failures with user privacy Skype had (storing ID's of motherboards without informing user, etc), and past of Skype original creator (spyware in original kaaza), + fact that skype is designed in clever way, that, above everything else, deny possibility to check what it *really* does, I'm surprised that anyone with at least single working brain cell is using it.

Google for "silver needle in the skype" for more technical info.

/Estel