Apple Adds Gatekeeper to Lion with OS X 10.7.5
According to the article the default setting is the middle one, which is "Mac app store and identified developers". I wonder what an identified developer is. I suspect that open source developers might not be "identified". Which means you won't be able to install software like vlc without changing the setting in the control panel. But before you could do that, you would have to know about gatekeeper first. I think 10.7.5 is going to cause a lot of headaches for a lot of people.