Based on the e-mail I got, I think several people missed Hank Nussbacher and Barry Greene's presentation at the last NANOG. http://www.nanog.org/mtg-0302/ppt/hank.pdf In the presentation they list several ASN and Network blocks in use, but not registered in any database. The US Department of Defense (AS 568) wins the prize for the most unregistered network blocks being announced. But they weren't alone. If you are going to start blackholing unregistered network blocks, I suggest checking if the current user has nuclear weapons. But once again, which ISP's aren't filtering?