Thanks for the pointer. The paper there is very interesting. I'm glad I regularly use Tor and NoScript.
You can have a look at https://panopticlick.eff.org/ for instance to see how information exposed by your browser makes you traceable. Every new "user-agent"-like feature will give more and more bits of uniqueness, at a certain point the aggregation of all this information would effectively allow to create a permanent serial number tied to an user or handful of users.
begin:vcard fn:Mark Callow n:Callow;Mark org:HI Corporation;Graphics Lab, Research & Development adr:Higashiyama 1-4-4, Meguro-ku;;Meguro Higashiyama Bldg 5F;Tokyo;;153-0043;Japan email;internet:firstname.lastname@example.org title:Chief Architect tel;work:+81 3 3710 9367 x228 tel;fax:+81 3 5773 8660 x-mozilla-html:TRUE url:http://www.hicorp.co.jp, http://www.mascotcapsule.com version:2.1 end:vcard