Henry, I think it’s Apple’s intention to block installation of apps outside the Mac App Store at some point. There are several ways they could do this. They could have installation blocked by default and allow users to switch on development mode. They could have people join the Mac OS X developer program and turn on development mode. They could make most Macs “appliances” like the iPad but have high-end macs have a way around the default security model. But I think they’re going to play up the security of the Mac App Store, and I don’t think they’ll provide the security of the Mac App Store for third-party application stores like Steam.