The debate is probably as old as the Open Source software development model: Which one is more secure: Open Source or shared source as we at Microsoft run it? I know that we could now enter a religious debate about that, which I do not want to as I do not really believe in the value of such debate.
However, it is always interesting to see who is looking how at this debate. Does it help security if everyone can see the code or does it help the attackers? We have a program which we call Government Security Program, giving governments under certain circumstances (e.g. protection of intellectual property) access to our source. Sometimes we have the debate with government officials whether having access to the code could allow an attacking government to get an advantage in the area or cyberwar or cyber espionage. Looking at that debate, OpenSource would even be worse as it means access for everybody.
Now, I just read this article: Open-Source Could Mean an Open Door for Hackers. It is about a paper looking at data from Intrusion Detection Systems and their finding is that flaws in open-source software tend to be attacked more quickly and more often than vulnerabilities in closed-source software. An interesting statement in the light that we know that there are more vulns in OpenSource software than in shared source and fairly often it is because of the lack of processes enforced to engineer security into the product from the beginning.
Another thing which is important to me is "As defenders get out their patches, the attackers have more incentive to move on to a different exploit," Ransbotham [the author of the paper] says. In other words, having a strong incident response (besides the engineering process) is at least as important.
This should be something the industry adopts. We made our engineering process called Security Development Lifecycle public and I think our incident response is wide known as well as being a best practice. So, something people should finally come to adopt