The Metropolitan Opera's production of Siegfried that premiered in the last season featured amazing 3D projections. They were developed by the Canadian firm Réalisations which had previously created effects for Cirque du Soleil and other theatre groups. The new technology allows projected 3D images on-stage to be seen without special eyewear. For instance, an opera singer might move inside a projection down the stairs of of a castle, which would appear three-dimensional to the audience. As the singer moves, the set around him or her would shift in what appears to be 3D. As far as I know those projections have not yet been used for a musical.