Forrester Research makes what is surely a controversial conclusion: Microsoft's Windows platform is a more secure computing platform than various Linux computing platforms. Forrester's counts the number of flaws in each platform over a given period of time and then analyzes how long it took for that flaw to be corrected. I've not got a link to the Forrester study but Computer Business Review reports that Forrester says Windows had fewer flaws than the Red Hat, Mandrake, or Debian distribution of Linux and Microsoft posted patches more quickly than the communities that develop those Linux distribution.
Interesting points which, if nothing else, may spur the development communities behind those Linux distributions to look at ways to speed up reaction time to security problems. On the other hand, I'm not so sure that you can make the blanket statement that one platform is more secure than the others without doing a qualitative analysis of the kind of flaws typical on each platform. Is one Windows flaw equal to one Red Hat flaw?
… And sure enough, the Linux vendors have now banded together to point out that there's a big difference between the quality of their flaws and the quality of a Windows flaw. The Slashdot gang are tearing into this