Revised answer: Yes
The longer answer gets a bit more involved. First of all, there is some level of protection built into OSX against malware called File Quarantine. There are limits to how much protection this provides compared with PC anti-virus and anti-malware products as it protects against known malware at the point where the malware is installed or run.
It is also limited by the frequency at which the OSX operating system is updated – OSX is typically updated once a week – unless you put off applying updates whereas a PC-style anti-virus product will typically update it’s virus definitions on an hourly basis. This would seem to make it totally inadequate, but OSX just doesn’t have as much malware as Windows.
There are a number of possible reasons for this including that OSX is inherently more secure and that OSX just doesn’t have enough of a market share for malware authors to bother with. The truth behind the lack of malware for OSX is only known to the malware authors, although it should be noted that OSX viruses do exist (as do Linux ones).
You could take the attitude that a flood of OSX malware is due any day now, and insist on running an anti-virus product in addition to the inbuilt protection OSX has. There are of course people warning that the flood of OSX malware is just around the corner, although they tend to be people connected to the anti-virus industry so are perhaps less than totally disinterested.
Of course if you have some seriously private data to protect, you should probably consider it. But most of us don’t work for the intelligence services, so can be a little less protected … for now. This of course can all change next month, next year, or sometime, so don’t take the word of this blog entry seriously especially if the date on it is a long time ago!
Of course now some time has passed, the situation has changed (with Flashback amongst others), so the answer is that yes you do need an anti-virus product. It is true that Apple has some built-in protection against Malware, but Apple is not an AV company and so they may well react too slowly to protect you.