Is antivirus software a necessity with the "rise" of MacDefender and others? I have heard broadly about the so-called "rise" of the Mac viruses. Apparently, there is a "whole new deluge" of Mac viruses. So far, I have gotten nothing that I can see.

But I'm beginning to wonder if this is a real danger and if I should take steps to protect my computer. I know that the Mac OS is very secure - that's part of why I use it - but is antivirus needed? I'd prefer not to have to use it - I've seen many PCs with antivirus that are slow because of it - and also because of the cost. Maybe I've just gotten lucky.

So: Is antivirus software a necessity with the "rise" of mac viruses? 

I'd count Apple/security experts as good sources.
I say no, and while MacDefender is not something I would ever install, it is an app you must install to be "infected" by it. Malware, crapware, bloatware or any other name it is not a virus which replicates itself without your knowledge or permission.

In a corporate environment you may be required to install some kind of email scanner because you will undoubtedly receive PC viruses in email from time to time - sometimes I have seen them arrive in hotmail etc. Having one may help stamp out the problem in the office but I have never encountered and never even read a reliable report of a Mac virus.

If you did want to install antivirus software, there's a good list in this AskDifferent answer - you might want to make an IT Manager happy, for instance.

Update April 2: This is also not a virus but a trojan - but it is worth knowing about.
Ars Technica report on a new Trojan targeting Mac OS X