At the moment, the software - which uses an encryption system called ZRTP - is in beta test and only works as an overlay to a small number of Internet telephony applications, but Zimmerman has ambitious plans, as he's submitted the protocol/standard to the relevant authorities.
That's the good news. The bad news is that it does rely on the SIP (Session Initiation Protocl) technology that most VoIP applications support. But the protocol could be incorporated into third-party VoIP applications if it becomes an industry standard.
And given Zimmerman's status in the encryption industry (fx: doffs hat in respect) that could happen.
So what's so good about the ZRTP technology?
Put simply, it's a blanket VoIP encryption system that exchanges the relevant keys across the IP transport stream in an encrypted format.
That means that sniffers and heuristic analysis packages can't decrypt a VoIP session in the fly, as is the case (allegedly) with Skype and other VoIP applications.
Sounds good eh? Watch this space...