Similar to this thread posted in June: Linux: More Vulnerable Than Windows?, a recent report says that the Linux operating system(s) are successfully hacked more than any other.

mi2g has been tracking data since 1995. Their database contains information on over 280,000 attacks. Based on their data, Linux led the pack during the month of August with 67% of the successful attacks being against Linux servers.

Of course, that doesn't change the fact that the world is scrambling and jumping through hoops to patch Microsoft vulnerabilities and stop the proliferation of threats such as Sobig and MSBlast. I guess the #1 target of virus / worm authors is still Microsoft.

For more details you can see this article on globetechnology.com: Linux is favourite hacker target: Study