I shared the original concept with Collin Jackson who developed the proof-of-concept code. The basic idea is one might want know if a Web user is in the Private Browsing mode in Safari and Firefox, the Incognito mode in Google Chrome, or the InPrivate mode for Internet Explorer 8. The way it works is by having someone visit a unique (never before seen) URL and then checking to see whether a link to that URL is treated as visited by CSS (standard color history hack). And if they haven't, then you know some privacy feature is actively blocking.
Definitely not anything super serious, but worth putting out there in case someone might have further ideas.