The idea is that Google is now indexing FOAF files and the XFN microformats to find and display publicly declared relationships between sites. Read the post on the Google Operating System blog for a good summary of how this works.

How linking strategies will evolve to include these self-declared links is unclear, but the idea that they will be indexed does make the effort seem worthwhile. Especially considering meta tags (keywords and descriptions) are ignored by most search engines, making other page elements more relevant. More indexed links theoretically should increase a given site’s popularity. If the “socially tagged” hyperlinks are considered more legitimate vs. meta tags by the search engines, they will indeed take off.