“On the face of it, open source software is more secure,” says Mitnick. “A lot of eyes are looking at the code. You'd think that with OSS, with more people looking at the code, you're more apt at finding security holes. But are enough people really interested?”
Though I don't think it's quite an issue of “are people interested” as compared to the average turnaround time for fixes, he raises a good point. The article is only a page long, but worth a read.