Dan York did a recent presentation on VoIP/UC security. I provided a link to the slides. I don't have the audio, I will post it if I find it. The title of the presentation is "Does anyone really give a ___ about VoIP security?".
Right now, I don't think most people really care too much about VoIP security. My take though (and I have said it efore), its not about VoIP/UC-specific security. It is and always has been about voice/communications security - attackers go after voice systems for the same reasons they have for a long time - namely, fraud/theft of service, DoS, harassment, social engineering, and more and more, for SPAM and phishing. Same old threats, it is just that VoIP/UC makes it easier and cheaper to execute these attacks.