OM Malik posted a long missive on Voice Over IP from Daniel Berninger. It is a must read if you are interested in this area.
Here is my favorite paragraph.
The competition between VoIP and the PSTN shapes up much like highways versus railroads. The operator of the PSTN and railroad own their transport network. VoIP companies and car companies do not. Railroads and the PSTN support a single type of usage. Highways and the Internet allow all user types to commingle. The emergence of highways empowered people to control many more aspects of their transportation needs rather than depend on the schedules and railroad routes available. The Internet accomplishes the same thing for communications. Automobiles and highways gave rise to an entirely separate industry and provided the basis for new types of commerce. The Internet offers the same promise, and corporate chieftains with traditional telecom assets find themselves in the same position as the railroad barons when Henry Ford got rolling.