Wow - (I saw this earlier and didn't know what it was either).
All I can say - is that it seems like it would involve a shader draped in front of/On the object(s) getting rendered on and someway to modulate the shader based on the projected light - (stomach pain) - way to much effort ....
Another way I thought would be some sort of ray casting - like sparky's - and either figuring out the position in the texture that is on the object and doing color math somehow - (blah - uv info(texture position etc - scale - yuck + object coords - stomach pain)
I'd make a work around. Like a ghosted object textured with the image or some such non-sense.
Rendering shadows is hard enough - a projector? OUCH!
I understand WHAT projection texturing is - and now that I do - I'll avoid it like the plauge!