Based on what I’ve read so far, anti-virus software for Linux is considered unnecessary.  I’m still researching why this notion exists, but the consensus seems to be that so long as you’re running Linux only you should be ok without anti-virus software.
In my case I’ve just finished installing Ubuntu as a proper parallel operating system that dual-boots with Windows XP.  In this case it does seem to be recommended that anti-virus software be installed; not as a precaution for Linux, but for Windows.  It’s feasible that a virus could enter …