Interesting article. I have argued for a long time that because Microsoft is the market leader in operating systems and a major competitor in the web browser market, they are going to proportionally get hit with the biggest number of threats like viruses, trojans, and malware, and hacking attempts on their software. This is not due to bad programming on their part, but the fact that they have such a huge market share. To their credit, they do a great job of releasing security updates on Patch Tuesday, as we have just seen this month re: Windows 7.
Look at it this way. We both live in a small town and there is a horse thief around. I have 90 horses and you have 10. Who is more likely to get robbed?
Not surprising to see Apple and/or AT&T are now taking a beating where they rule the roost - the mobile device area. There is no computer system in the world that is not susceptible to hacking or other malicious activity. Sometimes, no matter how great you take security precautions, all it takes is one determined (and possibly deranged) individual.
No OS is immune to threats. The market share Mike suggested is in my opinion the very reason why Windows gets a larger share of malware. What's different here is that Microsoft acknowledges the fact and works hard to release patches and updates to help protect users.
Now that they are the major marketshare holder in the mobile space, they'd better start taking security seriously. Security through obscurity won't work there. (And ironically, the more they use the "no malware" pitch to increase the marketshare of OSX, the more malware it's going to get)