As the market moves toward a 64-bit computing architecture, so do the virus writers. But they may find that being the first to write such viruses are going to put them at greater risk of being caught (which is good news for us!)
More information about the first 64 bit virus discovered by Symantec, labeled W64.Rugrat:
http://www.technewsworld.com/story/34077.html
A newer virus has just been found, called Shruggle. It tries to infect PE (Windows Portable Executables) files. However Shruggle is not harmful to 32 bit operating sytsems, unless 64 bit emulation is running.
More info about Shurggle can be found here:
http://www.symantec.com/avcenter/venc/data/w64.shruggle.1318.html