Interesting, ZDNet has an article about Debian / Ubuntu package management.
There are some interesting comments about why this wouldn't work in the Windows ecosystem. They're good arguments, but I think Apple just proved that a closely vetted app-store *does* work.
So, sure, an official M$ app-store wouldn't be the only place you could get Windows apps. Maybe Microsoft would only certify a handful of them given that there's an implied approval. Maybe M$ would have to be able to see the source-code and compile it themselves as part of their approval process. Nevertheless I think the app-store model would be incredibly valuable to Microsoft.