Wired reports that Verizon inserts a unique identifier into all HTTP requests going over its wireless network, subverting Do Not Track, private browsing sessions, using different browsers, or moving around their network. Verizon has an opt out page, but it only opts you out of having it being used by Verizon and its partners from targeting ads based on it. Obviously, anyone else seeing the headers are under no agreement to not use them to build a profile of you. There are anecdotal reports AT&T may be doing the same. Security researcher Kenneth White set up a page to check for this header with more information.