People have been arguing back and forth for (what seems like) centuries about Windows vs Linux/Unix. I am pooling Mac OS into Linux/Unix, because it is running on those platforms... it is just swaddled in expensive, sexy looking software.
Amidst the talks of Google promoting switching to anything but Windows, there was an article posted on ZDNet talking about an official Gentoo distro of Unreal containing a backdoor.
The scary part... its been there since November 2009! Now I have heard of Windows being vulnerable for long periods of time, but that is generally because users are slow to patch. When Microsoft is informed of a bug (it seems) like they attempt to take it seriously and address it. Of course there are many factors unknown-- like were they informed before and they just never acted on it? Also who knows how many issues are found in Windows but are quietly swept under the rug (think: Anti Trust) The article seems to infer that the admins inherently trust open source software therefor are actually LESS secure than their Windows counterparts...
While that remains to be seen, it is something to chew on... and with Macs gaining a new found market niche, it is only a mtter of time before hackers turn their gaze to the Apple world.