In a bid to squeeze out more cash through their existing service, one of the largest ISPs around has added themselves to the ranks of companies participating in controversial DNS-redirecting programs. Comcast has officially begun redirecting invalid DNS requests to their own "search portals," which are advertising pages intended to capture anyone who makes a typo or visits an invalid address into a web browser.
The technique is slowly being adopted by other ISPs around the world, and has been rife with controversy since the beginning. Essentially, anytime a user tries to visit an invalid web page, they are redirected to an advertising splash page rather than being told that the page is unavailable. As most other ISPs are doing, Comcast is offering an opt-out system - though there have been complaints that the process is not particularly friendly. Other ISPs around the world that have begun implementing this include Bell Canada, Verizon, and EarthLink.
Several years ago, Verisign, a major domain registrar, attempted to implement this. Their attempt received massive public outcry, and ultimately even ICANN stepped in and asked them to stop. Comcast has even gone a step beyond their cohorts, pushing for a new IETF standard that would make such DNS hijacking more legitimate.