The thing about 3D displays is that they need something to project the image ONTO. Sure, we have 3D cinemas and televisions and all that, but in all of those we are dealing with a 3D image being shown on a 2D surface.
To project a 3D image into thin air is beyond any technology we have today.
I actually saw a 3d Display about 6 years back at SIGGRAPH that wasn't projected, which was generated by shining crisscrossing lasers so that where they intersected, the air was turned into a plasma and glowed.
Granted, it was 1 color, about 1dpi, and has a refresh rate of maybe 10hz, but it was really cool.