I'm all for package managers (as other commenters pointed out, ubuntu already does this right), but I think when the packages being managed are commercial and not free/open-source, there needs to be some competition with the package manager itself. It's part of why Steam works so well- if it didn't have Impulse and GFWL and whatever else out there to compete with, Valve would have far less motivation to keep up the awesome sales and whatnot.

I'm not a Mac user, but I hope for the sake of those who are that Apple will allow 3rd party app stores (Cydia?) on the platform even though they haven't done so yet on their mobile devices.