Is antivirus software a necessity with the "rise" of MacDefender and others?
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