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depending on how the existing rotate feature is made it could be really easy to implement (thanks to Qt with QTransform). Maybe you just have to copy a bit code and change one parameter to Qt:XAxis / Qt::YAxis.
I think the most difficult part would be the UI.

In 2 weeks I have summer-holidays maybe I can find the time to have a look at it, but no warranties at all.

If anyone is looking for this to be actually useful and not just eye candy, I have a suggestion.

By looking at the picture, there are 3 surfaces to attach widgets to. A common desktop only has one. So just by implementing it, you have a theoretical 3 times as much space in this design, because you can fit more widgets on.

Take this one step forward, and let's link activities to each wall. That way, using this picture, you can have 3 activities displayed on the entire screen, instead of needing to swap between activities. Implementing this would not only give this 3D desktop a use, but maybe activities too

this wont ever come true, just because the sense of activities is another (its just not ready yet in 4.5 you will see much improvements)
Activities are thought as activities - as something you do on your computer. So you have activities Work, surfing, playing,... and you have EVERYWHERE just what you need for this activities. No Notifications from applications on other activities, no mails from your colleges when you're on a private activity,...
So you see Activities are NOT sites to get more widgets in it.

I'm glad to see some improvement, but I don't think it would be so bad to have a social activity right next to a work activity for group projects, or something of the sort.