This can be viewed as a good sign, since virus coders finally acknowledge the MAC as something with a user base worthy of their time.
This isn't a sign that OSX is going to go the way of windows though. The 2 factors we deal with in Viruses are:
1: Poorly designed software
2: Social Engineering
Windows is an excellent example of poor software design. In Outlook you can execute a virus simply by opening an email (seriously), in Linux and OSX you would have to open the email, download the attachment, and then execute it. In Linux, you would probably have to give the virus file executable permissions first also. Its not the same thing.
@nekatreven
You scoff at wymer100's comment, but he's right. You would have to be a complete MORON to download a virus like this, and then proceed to execute it. If I would have to first download a file, and then double click on it to execute it in Windows, I wouldn't bother with antivirus software. The problem in Windows, is often almost no user action is needed to let the computer become infected. Its not the same thing, and won't be until virus writers find ways to execute their code with less user action.
Disclaimer: I'm sounding like a MAC fanboy, but I like to think of myself as unbiased. Windows XP Pro, OSX, and Solaris at work, Unbuntu and Vista Premium at home, with a little photo work in OSX. Each OS has its strengths, but seriously, Linux and OSX are worlds more secure than Windows. Microsoft security vulnerabilities are so ridiculous, that in my organization users are not even allowed to have Outlook on their computers... not even for achieved email, or calendaring purposes.