They can be eavesdropped on by the people or organizations that own those computers, and they can be eavesdropped on by anyone who has successfully hacked into those computers. Do you want your competitors in on your conference calls?
Ultimately, Schneier concludes that the solution is twofold: encrypted VoIP and secure computers at the endpoints.
My first reaction to Schneier’s conclusions was, “We all want more secure operating systems, and for more than just VoIP.” My second reaction, though, is that the inherent insecurities of VoIP will eventually be addressed by the market once there’s enough demand.
http://arstechnica.com/news.ars/post/20060407-6552.html