Just as everyone else is saying... Rushing out and installing anti-virus software on your Mac isn't all that necessary. Most of the ones on the market for Macs are overpriced and/or terribly perform.
I've run Norton Anti-Virus v10 on my Macs for the last couple years as a way to protect my networked PCs, but it's mainly a 'warm-fuzzy'. If you want to make sure your Mac is squeaky clean of dormant viruses waiting to attack the first Windows machine they can get themselves on, then go for it. Make sure to find one that is Universal Binary if you've got an Intel Mac.
Also keep in mind if you run BootCamp or Parallels on your Intel Mac, you'll definitely want to have an Anti-Virus program running in them -- running Windows without Anti-Virus is just asking for trouble.