"One has to admit that Vista is arguably the most secure closed-source OS available on the market. Microsoft did do a good job at addressing the issues of previous Windows versions. Progress on all fronts has been achieved, and MS is probably better than any other closed-source software vendor when it comes to the the security of their products.
This makes it difficult for attackers. The cost of developing an exploit for Vista is significantly higher than for any previous versions.
As a result, I think that most of the security researchers will move on to greener pastures for a while. Why try to chase a difficult overflow out of Vista when you have Acrobat Reader installed, some Antivirus software with shoddy file parsing, and the latest ITunes ?
I expect only a small number of remotely exploitable vulnerabilities in Vista. We will see everybody else getting hammered though. But, for a while, there will be (relative) quiet and calm in Redmond."
http://blogs.technet.com/bluehat/archive/2007/09/28/vista-and-vigilance.aspx