Over on Blue Box, I uploaded on Friday what I consider one of the best overviews about SIP security that we've done: Blue Box Special Edition #20. I recorded the interview out at VoiceCon San Francisco in August and it's with Cullen Jennings who is a Distinguished Engineer at Cisco Systems, but more relevant to SIP is one of the Area Directors for the Real-time Applications and Infrastructure (RAI) area within the IETF. Basically all of the proposals for RFCs relating to SIP roll up under the RAI area. Cullen's also quite interested in and knowledgeable about security and in fact several of the security-related RFCs related to SIP include Cullen as one of the authors (as do a number of the current proposed Internet-Drafts).
So he knows his stuff... and being a frequent presenter, he's also good at distilling complex things down into more simple descriptions, so it was an enjoyable interview that I think you will also find quite educational. If you're working with SIP, or considering it, I'd highly recommend you listen to the show.