I went out this morning and bought a copy of Duncan Watts' Six Degrees because I wanted to catch up with the latest on the Small World phenomenon, particularly in respect to digital identity. While there is a resurgence of interests over the result of Stanley Milgram's experiment known as "Six Degrees of Separation", there hasn't been much discussion over how it applies to the digital identity problem domain. I think a robust self-organizing and fully distributed web of trust can be built using the Small World phenomenon. Relative success of PGP over PKI serves as a good indication that this approach deserves further study.
Here are some links to (very) few papers related to this. Note that these are recent (2001-2003) papers.
While my searches were mostly fruitless, I did run across an intriguing software named Six Degrees that offers some aspects of Universal Personal Proxy functionalities. It monitors e-mail traffic as well as desktop user activities to help the user find e-mail messages and files related to the currently selected object (called Focus). I like its use of the word Focus and how it integrates with Windows and Outlook. Its abuse of drag and drop was not, however. Check out an animated tour of Six Degrees here.