Mac OS X "Mountain Lion" and "Gatekeeper"
So the next version of OS X, "Mountain Lion," due out this summer introduces a new feature, Gatekeeper, which limits what apps you're allowed to install on your machine. By default, it only allows you to install apps acquired from the Mac App Store and apps that have been digitally signed, though you can disable it and allow it to install anything you want. Of course, this is almost certainly a transition strategy to a future where the only apps purchased from the App Store are allowed.
Shame really... I loved the Macs I've owned for the past 10 years or so because they had the best balance of Unix underneath with big time (relatively) commercial support on top and a really nice GUI to boot (at least compared to what was available at the time I got my first Mac). Now it's pretty much certain that my next main computer will not be a Mac and while I'll have the most basic MacBook Pro for iOS development purposes (I plan to write iOS apps to make a few bucks on the side), one of the first things I'm going to do is set it up to dual boot Windows.