The SIP working group within the ITEF has just published an Internet Draft describing SIP's vulnerability to relay attack and some possible mitigations. The attack method is via a "man-in-the-middle" approach whereby the attacker inserts himself in between the victim and their outbound proxy server, and initiates a session with the victim.
Given SIP's role as the common protocol within a unified communications architecture, it's important to be aware of security threats, especially as organizations widen their adoption of SIP-to-SIP extranet connectivity and SIP trunking for PSTN access.