Based on http://lwn.net/Articles/297366/ it would seem that more and more vulnerabilities are being reported. Using Mitre, the numbers for 2008 are 76, 2009 are 112, and 2010 is at 80 with 3 months to go. And that is just the known issues.
It may be a while before the tide turns. Since there are so many vulnerabilities, and their exposure frequency is increasing, it would make sense for the kernel to adopt some more defensive behaviors to make vulnerability exploitation harder. As a start, if we assume an unprivileged user will eventually have arbitrary write access to kernel memory, we can make choices about what is important to defend against within the kernel itself.