Simon St. Laurent has a great article on the Next Web. He traces several attempts to move from the HTML web to something more powerful – an XML Web or a Semantic Web. Neither of these efforts have had much traction. However, with the increasing adoption of AJAX, we are moving towards a very interactive web. The visible effect of AJAX is the user interface. In AJAX the client and server exchange fragments of XML to provide a rich interactive experience. Simon outlines other beneficial effects of server side AJAX.
As it turns out, the server-side support for AJAX can also frequently act as an interface to the server that can be used by other programs, without the need for a browser. Developers who craft smart APIs on their servers for use by AJAX-based web pages can then expose those APIs to other developers, getting the benefits of better interfaces for users who use web browsers to consume the data and for users who have their own custom programs consuming the data. Depending on how carefully the developer models AJAX transactions on traditional web HTTP transactions, these services even look a lot like the REST approach proposed earlier for web services.
I am a firm believer in the benefits of Data web or Semantic Web. However, anything that requires people change the way they currently build websites, is going to take a while to get adopted. Meanwhile, other efforts like Microformats, embedded RDF etc., are inching us towards a more semantic approach.