From the truly neat and truly scary department, via Slashdot a ZDNet article on a new way to track a PC on the Internet. A PhD student at Berkeley wrote a paper on a way to tell exactly which computers are making which Internet requests without knowledge of the user, regardless of the firewall or anonymising protections the computer and network may have. The technique involves monitoring the "time skews" of individual machines, which are the small differences in a computers internal clock.
I haven't read the whole paper, but I would presume that for this to work the tracker would need access to all of the packets the travel over a subsection of the network, which from what I know is not easy to do unless you own the network. (Presumably a Carnivore-like system would monitor this data, though). Second, on the privacy front, for someone to be able to peg activity coming from a machine to a _specific_ machine would require some way of matching the unique time skew to the machine, which can be trivial or extremely complicated depending on the level of network protections.
At any rate, it is scary to see what smart people can do. I've racked my brain to come up with a "good" use... but anything I come up with sounds straight out of DMCA or Patriot act. I take comfort in the thought that it's only a matter of time before someone comes up with a simple piece of software that will randomly vary the microseconds on the outgoing TCP packet timestamp and render the tracking technique moot.