With the release of Tiger, Apple will be making their EOF entity-relationship modelling approach available for use in all OSX apps.

Three data store formats will be available: binary, xml, and sql. This could help a lot with converting and wiring various app data into the greater semantic web. Apple will already be tapping Core Data for searching and sharing in Tiger:

The technically correct way to describe Core Data is as an object-graph management and persistence framework.

In down-to-earth terms, this means that Core Data organizes the application's model layer into a set of defined in-memory data objects. Core Data tracks changes to these objects and can reverse those changes on demand, such as when a user performs an undo command. Then, when it is time to save changes to your application's data, Core Data takes care of archiving the objects to a persistent store.

And, it saves data into regular files that users can manage with the Finder, search with Spotlight, backup to CD, and email to friends, family, and coworkers. Source: CocoaDev Discussion