Reconfiguring the Internet to make parts of it impossible to navigate anonymously.
Yes, this was a proposal considered - but apparently rejected, by the Pentagon. According to a NYT story,

"The Pentagon research agency that is exploring how to create a vast database of electronic transactions and analyze them for potential terrorist activity considered but rejected another surveillance idea: tagging Internet data with unique personal markers to make anonymous use of some parts of the Internet impossible."

Actually it sounds a lot like digital signatures to me. The plan was known as "eDNA", and it would mean a new version of the Internet, with enclaves where it would be impossible to be anonymous. So there would be "secure network highways", where a computer user would be identified, and private 'network alleyways' which would not require identification. The tendency, as we can all guess, would be for the identified bit to swallow up the non-identified bit - except for those, probably, who are either (a) privacy fanatics (as Alan Westin used to call them), or (b) criminals. For everyone else, if the information is even vaguely useful, not hard to collect, and if people can't be bothered routing around it (very common in this area where people don't tend to value privacy until it's gone), it will just latch on to the secure bit. I'm relieved to hear it's not being pursued. I don't want to feel I'm being monitored that way (I feel that enough already, given that I use a university server). Code as law, LessigReidenberg/ style, indeed.

More quoting:
A description of the eDNA proposal that was sent to the 18 workshop participants read in part: "We envisage that all network and client resources will maintain traces of user eDNA so that the user can be uniquely identified as having visited a Web site, having started a process or having sent a packet. This way, the resources and those who use them form a virtual `crime scene' that contains evidence about the identity of the users, much the same way as a real crime scene contains DNA traces of people."