it's just the case of Virut (the only widespread virus, which has a 64bit variant and needs a special algorithmic detection)... as you probably know the x64 assembler is different from x86 assembler and that makes also the detection approach slightly different (maybe a bit harder to keep the track).. there's already a detection of 64bit virut (Win64:Virtob), but it must be updated to cover the few newest variants...
