Somewhat more key in this debate is the idea of prioritizing types of traffic and not just particular sites. A lot of ISPs are phone companies -- Verizon, Qwest, etc. Even the cable companies now provide phone service over cable.
Now let's say you've chosen to go with Voice over IP -- you have your phone delivered over the Internet line rather than the Internet delivered over the phone line. If the ISP is also a phone company, they may deprioritize VoIP traffic in order to encourage you to drop VoIP and get their phone service instead.
