In a report today, Google announced that they are dropping Microsoft as their operating system, opting for Mac OS or Linux, citing security concerns as Chinese hackers tried to break in Google and other large companies in January of this year. Back then, Google used it as an excuse to pull out of China. I think the only concerns Google has is to knock Microsoft at this time because of the pending release of MS cloud computing applications, Office 2010, and Google's, soon-to-be-released, operating system based on the Chrome browser.
I attended a Google Apps seminar last week where they ensured the audience that they take their security very seriously and even had gone to the length of hiring ex-Pentagon staffers to help in that regard. They said that their security is much tougher and much harder to break in to then most other organizations. Good thing because they have millions of people using their Gmail, Docs and their other cloud applications. A security breach would be devastating for Google and its users. On the other hand, 9 out of 10 computers in the world are running Microsoft's operating systems. If there was a serious computer threat from China, I'm sure that many other large corporations would be leaving Microsoft or they would be find a way to block the threat.
Although Google is probably a huge target for hackers, I think that their switch out of Microsoft is more of a competitive move rather than a security concern.