In Dec last year, I added support for an external DHT in my SIPpeer application, i.e., the SIP-using-P2P model. A technical report describes the details. In particular I used OpenDHT and built signing and encryption on top of it to allow secure P2P SIP with identity protection. The advantage of using a managed external P2P network is that we can safely assume that OpenDHT nodes are trusted in the sense that they perform DHT routing correctly. Thus the security problem is partly solved, and the identity protection can be done by issuing a certificate to the user if he/she has an email address.
Then in Jan and Feb this year, I wrote a Tcl based SIPpeer connector for OpenDHT, and with the help of Xiaotao Wu, added it to our SIP user agent, sipc. We are trying to see if we can make it opensource or at least freely downloadable for non academic use also. But first I will need to fix some of the usability issues. I will keep the current status posted here on time to time.