Quoted from http://www.securitydistro.com/index.php?option=com_content&task=view&id=305:
SecurityDistro - BackTrack 3 Beta Released
BackTrack 3 Beta ISO version (Stripped Down - 700 mb)
http://www.offensive-security.com/bt3b141207.iso.torrent
BackTrack 3 Beta USB version (946 mb)
http://www.offensive-security.com/bt3b141207.rar.torrent