You may disagree with this statement I made earlier today:
Linux on the desktop will never be as polished as OS X, or even Windows, until some company pours hundreds of thousands of dollars (if not more) into building a usable UI on top of it. However, any company that dumps that much into it isn't likely to make those contributions open source.
It is very possible to make Linux desktop-ready. OS X is fundamentally a BSD-clone with a lot of money and time poured into the user interface. So given some vast amount of time and money, it should be possible to do the same thing with Linux.
I could be wrong but: Most organizations that focus on the open source aspects of Linux (well, properly, GNU/Linux and other stuff) tend to not have a lot of money. So providing the required amount of money will probably fall to a company. And a company that is making a such a significant investment is going to want to capitalize on that. Pouring vast amounts of time and money into something and then giving it away is a losing strategy.
It is possible for the software foundations to fund the development but it will take significantly longer. The added time will ensure that desktop Linux UI would always trail the corporate offerings (Windows, OS X, etc.) because it will have never caught up (assuming the corporate offerings do not stagnate).