Having your Skype client open on your PC also means that your network can be used for relaying other Skype calls if the system detects incoming TCP/UDP connections can go through your NAT firewall (e.g. due to UPnP opening a few ports). It seems this doesn't happen very often as I haven't observed a relay session over my PC until recently, when I suddenly had a 200 kbit/s data stream in both direction running over my PC for several minutes.
Wireshark is always close at hand so I could quickly confirm that the data stream was indeed to and from the Skype client. A Reverse DNS lookup then revealed that the call was between two other PCs in Germany in networks of different providers. That probably means that the DSL routers in those networks do not have UPnP enabled, otherwise the IP packets could have been exchanged directly between the two parties. Also interesting to note was that the relaying went over my Wi-Fi. So unknowingly to those on the call their data packets went over a wireless link at least twice, probably even four times if you assume that both parties used Wi-Fi at their end as well.