Perhaps a better question is "what is 3-dimensional content?" After Blaise Aguera y Arcas wow'd 'em at Microsoft with Seadragon (now Photosynth), the next question is "if we can do that with pictures, can we do that with content [a.k.a. knowledge]?
How can the Internet provide us with a "depth of information"? In other words, is it possible to create a deeper way of seeing information? Not via simple link lists such as with wikipedia (which is actually done manually), but by using the power of the machine.
Web 2.o sites such as del.icio.us and Microsoft's Listas create 3-dimensional content typographically with differing font sizes in a technique called "clouds". Here, the human community contributes to the knowledge base through the popularity of certain sites.
But the Internet, as a vast repository of data, needs more to be a vast provider of information.
A service currently under development is trueknowledge.com. Take a look at this video from techcrunch.com. They have developed a search engine that not only locates a number of relevant sites, but also answers direct questions. Check it out.
