A professor at the Buenos Aires Institute of Technology proposes a means of projecting 3D floating objects taking up actual "3D" space-- using 10 regular LCD projectors and a smoke machine.
Calling the system a "volumetric projector," Marc Ressl has a video where he steps into and walks around a 3D shape the system projects onto a bank of smoke.
Ressl admits the video serves mostly as a proof-of-concept, using a too-low number of projectors (he says the individual light rays become invisible and the contrast improves if one uses 100+ projectors) and a "not well-suited" fog machine.
He is now using the video and companion blog to find whether anyone sees commercial potential in the prototype-- and get some funding in the process.