Rear projection is definitely possible. Most projectors have settings to reverse the image. You need a transparent screen, but many companies sell that as well. The biggest thing you need is space to mount the projector behind the screen. Projectors require a fairly sizeable throw distance. Figure 12-14' for a 120" image.
The good news is that it doesn't need to be straight back. You can use a mirror mounted at a 45 degree angle to direct the picture toward the screen. However, all of this is fairly complicated and will cost more than front projection. This is why most folks will just go for a front projection system.
You might want to pop on over to the screen forum. There are several good threads there on this topic.