kantbleveit, I just addressed the question in my last note to Weby.
To answer your question simplistically, yes and no.
Yes, more viruses and hacks will pop up. The depth and breadth will not be as great because there are less vulnerabilities.
No, viruses and hacks will not run rampantly and virally. That is because the open source space operates in a customer service environment (culture) that reacts very quickly to patching the few weaknesses that are discovered.