Well, it happened. According to this Technet article Alureon evolved to attack 64bit version of Windows
http://blogs.technet.com/b/mmpc/archive/2010/08/27/alureon-evolves-to-64-bit.aspx
Does Avast protect against this ?
I think this has been the case for a while now.
Since avast detects Alureon in the 32bit OS my guess is it would detect it in 64bit OSes. However, there are slightly different Alureon variants as it continues to evolve so it is still likely to be a catch-up game.
The main one is of prevention not letting it get established.