MACs are not immune to viruses. And they certainly aren't immune to installing trojans. (Things you download and install, or receive in email attachments, which is the most common way of getting a virus.)
There have been several visuses that attack unix systems, and the underlying OS in the MAC is a unix-based operating system.
There just haven't been any viruses that specifically attack MACs yet. The one virus that was mentioned was theoretical only and required you to click on something to install it. It was more of a trojan/virus than a regular virus that can attack a computer without you doing anything specifically.
There are AV products available for the MAC, but most people say you don't need them. I think MACs ship with a built-in firewall, but you may have to enable it. Certainly if you travel with a mac notebook, you'll want to enable the firewall. If you just use it at home behind a router, you're generally already protected by your router.
No matter if you are a MAC or PC user, you still need to be very careful when opening email attachments or clicking on arbitrary web sites that might try to install something on your computer.
I don't own any MACs, but I think they are a great choice. They are much easier to use than PCs. And with Vista being the only choice for many new computers, MACs are a much better choice for most people. The choice and preference are definitely subjective, as is this last paragraph.
-David