A good article on O'Reilly about SPARQL, a SQL like query language for RDF. If you can get people publish RDF representations of their data, then you can query them using a common language.
Okay, so SPARQL gives the SW and Web 2.0 a common data manipulation language in the form of expressive query against the RDF data model. Web 2.0 needs something exactly like that. (Imagine the horror of trying to get all of these totally uncoordinated Web 2.0 services and apps to support the same SQL queries? That's completely impossible. It will never happen. It may be hard to get them all to map SPARQL into how they really store data. It may never happen, in fact. But it could happen, and it will long before everyone uses the same RDBS schema.)
Via: Tim Bray