You do need to turn off signature checking in the security settings, so not completely out of the box. Given that the application store can be changed to black

You do need to turn off signature checking in the security settings, so not completely out of the box. Given that the application store can be changed to blacklist a certain app, it is not unlikely that the next step is similar changes to the apk installer. It should also be noted that there are no guarantees that signature checking is optional in all future builds of Android. While there are differences between the iPhone and Android business models, they should also not be overstated. My personal guess is that they will drift closer over time rather than farther apart.