Wired has a nice article about two companies producing new types of projection methods. IO2Technology impresses viewers with an image projected on vaporized water. The nice thing it track movements of the viewer and allows screen control with your hands - a bit like "Minority Report". According to the pictures on their website the resolution and luminosity of the pictures is quite low. But it's a start...
The other one mentioned is Fogscreen which allows you to walk through the display as well. It also uses vaporized water to build a projection screen, but seems to reach much better luminosity. These projection methods make a very good eye catcher. Let's see who will be advertising first with it...