I don't know about that. Anybody can package any software they want in a file that will be read and installed by the package manager. If you want to have something installed on Ubuntu, just package it in a proper .deb file and you can distribute that file any way you want. Once your user/customer gets a hold of the .deb file, all they need to do is double click it and your software gets installed by the package manager. There is no review process, there are no rejections, nobody takes a cut of your fees.

And the user can add 3rd party repositories.

My third-party Mac repository is Google.

Why use Google if one can be lazy and use MacPorts ;)?

And going forward with the AppStore, users can still download and install from a .dmg or a .pkg as they do now.

Your .deb could satisfy dependency requirements, but it won't be able to take advantage of "check for updates" functionality of the underlying package management ystem.

The advantage of the App Store is that Apple will make sure nobody submits an app with a dependency on locusts.

http://www.xkcd.com/797/