1 Ever hear of the Pepper's Ghost illusion?  That's essentially how the Musion Eyeliner system, which is what's used in Gorillaz concerts, works.  It's not frosted glass, but a thin mylar-like film.  It's great for covering large areas, like a concert stage, but really can't be used outdoors or anywhere there's much of a breeze (the film is so light that it'll move).

I'd be willing to bet that you're right about the Hatsune Miku concert using the same technology, though.

2  There's no need for an illusion. She's gonna manifest from herself and lead us into the age of aquarius.

She'll achieve true corporeal form just like she does here...where she shows that she favors Mac users...

http://www.youtube.com/watch?v=nQxzumHN4ps&feature=player_embedded

...only she'll be...
...ummm...
...bigger!

3 I didn't know that Miku was available as a doll for that program. (And I do know that the program is available for the PC, not just for the Mac.)

4 You're pretty much right about it being like the Gorillaz illusion.  There have been some live vocaloid concerts already in Japan.

http://www.youtube.com/watch?v=iNrPHx60Q2Y

I think they use a glass sheet instead of the suggested mylar film, as it doesn't move much.  The bad thing is thet all those glowsticks reflect on the glass, adding a lot of visual noise to the illusion.

5

Man, that really looks good!

Of course, it would fail if the rendering wasn't up to snuff, but that too is a conquered problem. And the models they used are nice looking.