It appears that some major ISP's, such as AT&T, may filter the traffic that passes through their networks in order to eliminate illegal file-sharing.
Here's an excerpt from "MPAA Head Wants Deeper Relationship (Read: Content Filtering) with ISPs":
Rather amazingly, given the money and time that will be required to implement such a system, AT&T has agreed to start filtering content at some mysterious point in the future. Other ISPs could well follow suit, as most of the major networks are owned by or affiliated with companies that also have a voracious need for content (just think of how both cable companies and telcos like AT&T and Verizon need access to channels for their various TV offerings, if you need an example). The companies want to keep on good terms with content owners, but there may also be some legitimate concern about the impact illicit traffic has on their networks. Cracking down on illegal file-sharing—should that prove to be technically possible—could help with both of these issues.
Source: Anderson, Nate. "MPAA Head Wants Deeper Relationship (Read: Content Filtering) with ISPs." Ars Technica, 19 September 2007.