Well it's here folks.
<EDIT>
I have erroneously jumped to conclusions and accepted the common perspective that this piece of software that self-installs is a "virus" but upon further inspection, technically speaking, it is NOT a virus because it does not self-replicate. Furthermore, this piece of software has not spread rapidly and has been contained rather quickly by Apple's quick response through Software Update.
I have left the remaining portion of this entry here for informational purposes only but thought it was necessary to clarify things and to keep things in perspective. So far, if your Mac is becoming non-responsive it is safer to assume that your Mac does not have a virus but instead has some other kind of problem.
</EDIT>
I have already removed the first Mac Virus from 3 computers. It was released last week. It is usually downloaded from very shady websites. It attempts to fool the user into submitting their credit card number for an anti-virus program which is actually the virus itself. The payload does not seem to be critical to the computer but users should be aware of this. ClamXav catches this as does the latest OS update from Apple. It is definitely suggested that everyone update their OS to the very latest of Apple's patches.
For those companies that have proxies and filters on their networks, the threat is minimized even more. However, this trojan does replicate itself so it is likely that this will become more available on less shady sites eventually.
This is a classic scenario with Windows machines. By being vigilant, maybe we can keep it that way.
Other tips:
1- When using Safari, turn OFF the option to "Automatically open safe files" under Safari > Preferences.
2- Delete any files that have names like MacDefender, MacGuard, or MacSecurity.
3- Do not open zip files that are unknown.
For more info: http://www.macrumors.com/2011/05/02/new-macdefender-malware-threat-for-mac-os-x/
UPDATE: The Virus has already been re-released under a different name which Apple's update does NOT recognize. This is a cat and mouse game. The most effective way to avoid this virus is to be smart, don't open anything that is unknown to you and definitely- DO NOT HAND OVER YOUR CREDIT CARD INFORMATION!