Historically Apple hasn't enabled skinning or theming in OS X, beyond changing some colors and your desktop photo. I imagine they'll continue that trend.
I would actually like to see themes and skins. I never understood why that should be so big of a deal. As much as gray is a useful color when I deal with photography, it becomes boring and I'd like to be able to have a bit more to choose from than blue and graphite. In this sense, Windows makes life a lot easier.
That's why it's not coming to OS X
That's an example of going overboard. Although, to whoever made that skin, maybe they liked the way it looks. If anything more choice would bring more customers to Apple. People like customization, it makes them feel special
I hope to god apple never. ever. ever. allows theming. All the themes for Windows look ugly, including the standard ones.... Im sorry, but if you let the Mac be themed, you end up with ugly inelegant desktops - and apple is all about beauty, simplicity and elegance. Not about user customization.