No need for virus protection for now. If you want to protect PC users from spreading an infected email attachment, try
ClamXav. It's free and it uses the open source ClamAV engine that is built into the server editions of OS X.
To greatly decrease your chances of getting a trojan or virus, follow these rules:
1. Do not download from shady sites and P2P networks. The new Mac trojans that popped up this year infected those who downloaded pirated copies of iWork 09 and Photoshop CS4. They used social engineering to trick people to install the trojan. The trojan didn't just install itself.
2. Do not run as an administrator. Go into System Prefs>Accounts and make another Admin account. Then log into that one and change your account to Standard. Every time you install an update or app, you will have to authenticate the task. This keeps rouge software from installing itself if hackers/crackers find a way to spread viruses.
3. If you use Safari, disable "Open 'safe' files after downloading". Again, in case virus/trojan writers find a way to exploit this feature.
4. Optional: Turn the built-in firewall to only allow essential services. Usually routers have adequate firewalls, but adding software firewalls add an extra layer of security.
5. Optional: Run software like
Little Snitch or
WaterRoof as an addition to the firewall. Little Snitch monitors outgoing connections and WaterRoof gives you a nice GUI to set IPFW rules.