If your Data Space was a Solar System, your personal Identity would be the Sun. I say this because your Identity is the conduit (access mechanism) to your data graph; the data you generate from various application interaction activities such as: Blogging, Bookmarking, Photo Sharing, Feed Aggregation etc.

Daniel Lewis has just published a nice blog post titled: The Data Space Philosophy, that puts the underlying Data Space concept in perspective.

The Linked Data Web is a Giant Global Graph of Data Spaces (meshes of data and identity exposed by graphs connecting data and identity)

Data Portability ultimately depends on platforms that provide unobtrusive generation of Linked Data (for data referencing) alongside support for a plethora of industry standard data formats -- which is what OpenLink Data Spaces has been about for a very long time :-)