Firstly, I believe there is a possibility of a virus affecting a camera - albeit very, very unlikely. Any item that uses software or flash based firmware is potentially vulnerable to a virus - there are, for instance, growing concerns regarding viruses that can infect cellphones and PDA's. Although, it seems unlikely that someone would bother to design a virus to attack a camera, I would say that it is not out of the question.
Also, I'm not sure that infecting your memory card would require you to knowingly copy the virus over. Many viruses replicate themselves to any 'drive' connected to the infected PC, and this would likely include virtual drives such as CF-Cards. Again this might be unlikely, but it is yet another strong argument for having an regularly updated virus scanner on all your PCs and/or Macs.