Jonathan Mayer writes:
Over the past couple of days, there’s been an outpouring of concern about Verizon’s advertising practices. Verizon Wireless is injecting a unique identifier into web requests, as data transits the network. On my phone, for example, here’s the extra HTTP header.1
Read more on Web Policy.
Update: See also this article by Robert Lemos on Ars Technica: Verizon Wireless injects identifiers that link its users to Web requests