Comp with single Ai file, 20 layers.
Select all, change them all to 3d layers.
Select single layer, move Z position, nothing happens.
Ok then...Precomp all layers into new comp.
I can see the results in the new precomp but still NOT in the original layer making it impossible to see what I am doing.
I see another person had this same problem with no solution.
Oh, I am on a macbook pro with an ATI card, I know...they are pretty lame machines but I can't afford to get a pc right now.
Should be working fine. Are you sure you're looking at the active camera and not the front view. If I duplicate your workflow (which I just did with an anaimation that has 270 illustrator layers) on an old MacBook Pro, it worked fine.
What is the compositon 3D renderer set to? Check that it's using the standard 3D. And of course the usual: Verify that there's no layer styles, adjustment layers or something else whacky that may break 3D rendering order.