Ummm... Thinking out loud..
Just some thoughts....
- iTunes runs under Mac and Windows.
- iTunes store accessible under Mac and Windows.
- iPhone can sync and live reasonably well in a Windows environment.
- Sandboxed development environment for existing iPhone apps (no private API).
- Meant they basically ALL ran on iPad on day #1 without problems.
- Restrictions apparently not a real problem -> 300,000 apps and counting.
- Applications can run on Intel hardware via SDK emulator at some level.
Now, I wouldn't expect existing iOS apps to run right away on the Mac App Store as there are a lot of hardware components that are missing (touch, GPS, accelerometer, etc) that would make this not so great.
However, what is stopping them from releasing an App store with a client that also runs under Windows? You launch an App for the store, much like iTunes, and from that point onward, you're running in a sandbox (a curated experience entirely) potentially with Apple providing all of the APIs and the translation layer. Voila, 1 BILLION desktops available to your apps, and every single one of the hundreds of millions of iTunes customers are now your customers, by virtue of all of them being iTunes users.
Sure, it doesn't help people writing software that accesses hardware directly, but would explain the restriction on extensions and drivers, etc.