Qwest.net has become the second major North American ISP to block Internet Access to infected computers. http://blogs.pcmag.com/securitywatch/2007/10/qwest_takes_action_against_inf.php#comments
Cogeco Cable of Ontario, Canada has been doing this successfully for a while already.
Of note is that many ISPs are now throttling speeds for users of large file-sharing networks also. Comcast is of note here for throttling Western Washington users to as slow as 56K when an active BitTorrent session is running. Makes it faster to download those Linux ISOs off of an FTP server again, talk about going backwards technologically. I do understand though that all the ISPs are feeling the heat from RIAA/MPAA to reduce the amount of illicit traffic trading illegal files, and must say that I support them in this as I have many friends in the entertainment industry.