I advise you to ignore those people who tell you not to install virus software, Sir Loin Steak. OSX is just another flavor of unix, and unix isn't perfect.
Rather, consider it a calculated risc.
At one point or another, viruses will show up on the Mac platform.
It's not today, it may not be this year, but eventually it'll happen.
As always, best practices apply.
OS X's built in firewall and rather closed-by-default state are good things.
The security model is another.
Realtime scanning isn't worth it ATM unless you're in an office environment and want to help protect your colleagues.
On-demand scanning scheduled to run every day or week is another thing, and at this point in time, the free clients are easily as good as the proprietary ones.
Personally, I don't want to waste 5-20% resources on constantly scanning for viruses or malware -- I just make sure to use software I trust not to have been tampered with, and I use TimeMachine for backups.
The important stuff on my computers is photos, videos and documents I've written (be they code, articles, guides or technical specifications).
If my applications get tampered with (the definition of a virus is that it self replicates), I can easily reinstall both the OS and them and be back up in no time at all.
If I were to install something from an unreliable source, I can see any eventual trojans running in Activity Monitor.
I also subscribe to various security related mailinglists (part of my job) to keep track of what exploits are out there and the status of the fixes.
Like I said, it's a calculated risk -- but right now I can't be bothered paying $100 - $400 for having realtime virus scanning (5-20% of the computer's purchase price).
When viruses start popping up and the threat is more credible I might change my attitude, but right now it's not
necessary to run antivirus on the Mac.