On my Retina iMac the menu bar is light but the Dock is transparent & dark. This is partly to match the wallpaper. If OS X would have a full dark mode I would be tempted to use it more often.
...and bringing back the menubar shadow!
Fingers crossed for 10.12, if there's a "full dark mode" in the upcoming OS, that would actually be kind of awesome 🙂
I can tell you for certain that there won't be a menubar shadow as long as the "flat" GUI is in place with the operating system. It would simply look out of place. I would go so far as to say that "never" is when it will return.
As far as a full dark mode. I'm with you. I would love it. But I also don't think we'll ever see it. It presents a lot of problems not only for the user, but for developers as well. Apple and Devs would have to re-consider the UI of every app so that it works in dark or light mode.