There’s no doubt that iOS is where the developer mojo is. Not just Apple’s developer mojo, but the industry’s. It’s the hottest platform in the world, period. And I do think Apple sees the upcoming Mac App Store as an opportunity for iOS developers who’ve never written apps for the Mac to start. But I don’t think iPad apps are ever going to run on the Mac as-is, without any change or adaptation to account for the very different input methods. This isn’t about ARM vs. x86 code generation, or development frameworks. It’s about the fact that direct manipulation on a touchscreen is fundamentally different than moving a mouse cursor via a touchpad.
Gruber’s right — running apps that are built for finger-based inputs on a system driven by a trackpad or mouse would suck. But can you imagine what will happen when developers figure out how to solve such issues?