Is Open Source Insecure?

Lumpy writes: "Lately there has been a HUGE push by Certified Microsoft Professionals and their companies to call clients and warn them of the dangers of open source. This week I received calls from 4 different customers that they were warned that they are dangerously insecure because they run Open Source Operating systems or Software because "anyone can read the code and hack you with ease" they are being told. Other colleagues in the area also have noticed this about 3 Microsoft Partners or so they claim have been going out of their way to strike fear of OSS in companies that respond with "yes we use Open source or Linux" when the sales call comes in. I know this is simply a sales tactic by these companies that will remain nameless, but how do I fix the damage caused by these sales tactics? I have several customers that now want more than my word about the security of the systems that have worked for them flawlessly for over 5-6 years now with minimal expense outside of upgrades and patching for security. Does anyone have a good plan or sources of reliable information that can be used to inform the customer?"