It also does not rely on SIP signaling for the key management, and in fact does not rely on any servers at all. It performs its key agreements and key management in a purely peer-to-peer manner over the RTP packet stream. It interoperates with any standard SIP phone, but naturally only encrypts the call if you are calling another Zfone client.
This new protocol has been submitted to the IETF as a proposal for a public standard, to enable interoperability of SIP endpoints from different vendors.
In keeping with the long-standing PGP tradition, the source code is also available to download for peer review.
The Zfone public beta is available today for Mac OS X and Linux. The Windows XP version will be available in mid-April.