I recognize the similarity between phone calls and Tor; however, I too think there is a difference in the expectation of privacy.
I like your analogy with aerial surveillance, but partially because I think you are wrong. It is my understanding that aerial views available to the public, such as those available to a commercial helicopter, do not require a warrant. However, if the police go beyond public capabilities with high zoom cameras it may require a warrant. Similarly, if the state goes beyond the capabilities of the public in obtaining my IP address, they should require a warrant. The question then becomes, "Does the CMU project constitute knowledge generally available to the public?" I would argue the negative, but I could see it answered either way.
It doesn't need to be available to the public, only available to third parties. There are a finite number of people (either 3 or 6) that together have all the data needed to identify the user.
I like your analogy with aerial surveillance, but partially because I think you are wrong. It is my understanding that aerial views available to the public, such as those available to a commercial helicopter, do not require a warrant. However, if the police go beyond public capabilities with high zoom cameras it may require a warrant. Similarly, if the state goes beyond the capabilities of the public in obtaining my IP address, they should require a warrant. The question then becomes, "Does the CMU project constitute knowledge generally available to the public?" I would argue the negative, but I could see it answered either way.