The short answer is this: No, CentOS 5.6 is inherently no more or less secure than any other modern supported operating system. The long answer is a bit more complicated.
CentOS is the "Community" release of RedHat Enterprise Linux (RHEL). The differences between the two are fairly small so you can think of them as functionally equivalent, see the Wikipedia page for more information. The relationship between them is best described as a workflow; as RedHat releases packages, the CentOS maintainers full down the source rpms (ignoring any packages they need to for licensing reasons), rebrand and repackage for their own releases while maintaining version numbers. This history and workflow is important to your question.
RedHat's business is selling a stable product with support. Each major release of RedHat Linux is defined by a single kernel version, and often the versions of big packages such as Apache remain fairly stable. The RHEL team does, however, backport relevant security fixes. These backports are what allow the RHEL 5.x series to use kernel version 2.6.18 but still be patched. Due to the above relationship, this work also translates back to CentOS.
All that being said, given the work required to translate the packages one must expect a time lag between a RHEL release and a CentOS release. Normally the lag is fairly small, occasionally you will run into the situation that we had earlier this year when there was a couple of month lag between RHEL 5.6 and CentOS 5.6.
Going back to your question: Is CentOS a secure distribution? Is pinot grigio a good wine? How good is your admin? As I said in the beginning, "CentOS is inherently no more or less secure than any other modern, supported operating system." Any system can be hacked into. Just make sure you are following good practices and all of your other layers are in place.