I am really looking forward to the Win Phone 8 SDK. I have done some development myself previously. I tried an SDK, called Ozeki SIP SDK and according to the following guide made a VoIP app for WP7. In case some of you decides on developing a VoIP app for WP7 it might you some good hints (http://www.voip-sip-sdk.com/p_511-windows-phone-platform-voip.html). I am also interested what changes will the Win Phone 8 SDK bring.