There has been this flurry of posts on Web 3.0 now that Web 2.0 (whatever that was) has mainstreamed. Its always amused me to apply version numbers to something that is changing all the time with new services and sites appearing daily, but that's marketing for ya!
What is really interesting to me in all of this "let's jump to Web 3.0" is that people just assume its going to be a simple jump in versions. In reality, semantics and ontologies are not easy...not easy at all. If you want to catch a glimpse of how the W3C defines the semantic web look at the OWL Web Ontology Language Reference.
Relational databases can be hard for folks to get, but descriptive logics...yeah. Using ontologies can be pretty straightforward but I will say that defining them can be a pain in the rear end. Programmers think that its about class heirarchies (like in programming) but its really not. Individuals (objects), via inference, can enter and exit class membership based on the facts you know about them. Classes have union, compliments and disjoint definitions.
Properties are fully namespaced and those properties have...properties. Think of this...you define a namespace#hasChild property which is the inverse of a namespace#hasParent property. Pretty easy to figure that out. Now is namespace#hasHusband the inverse of #namespace#hasWife? Damn...a gray area (in many people's minds).
This brings me to the main issue that people are going to run into when trying to work with ontologies and semantics on the Web and that is gray areas. We have many of them and the current crop of "social" Web sites allow the human to deal with that gray area (using the nuance of human language). In the world of ontologies systems have to do that, and its very, very hard.
All that said, there is real power to ontologies and semantics captured and used by systems. The key to these systems IMHO is that they are constructed on some real ontological and semantically rich foundations (not just a relational data model), and all that crap is completely hidden from humans. Humans build up and leverage stuff on that rich foundation without ever knowing about it. They may feel its there, but don't have to understand it to use it. An environment that allows this for normal folks (on a continual basis) is one of our goals with indi. I really don't like to jump on bandwaggons but it seems the wagons are coming to us. Its nice when that happens, welcome to the parade.