The Web is entering a new phase of evolution. There has been much debate recently about what to call this new phase. Some would prefer to not name it all, while others suggest continuing to call it "Web 2.0". However, this new phase of evolution has quite a different focus from what Web 2.0 has come to mean.
The threshold to the third-generation Web will be crossed in 2007. At this juncture the focus of innovation will start shift back from front-end improvements towards back-end infrastructure level upgrades to the Web. This cycle will continue for five to ten years, and will result in making the Web more connected, more open, and more intelligent. It will transform the Web from a network of separately siloed applications and content repositories to a more seamless and interoperable whole...
Because the focus of the third-generation Web is quite different from that of Web 2.0, this new generation of the Web probably does deserve its own name. In keeping with the naming convention established by labeling the second generation of the Web as Web 2.0, I agree with John Markoff that this third-generation of the Web could be called Web 3.0.
Also worth reading on this subject is Nova Spivack's 'The Meaning and Future of the Semantic Web'