(image comes after... it's weird that I cant have any image at my first post.... )
we've hacked a bottle
stuck in a vibration sensor, a light sensor, a motor and linked them all to arduino board
audiences can shake the bottle
the force, frequency and period of shaking determines the color of the "outcome as an emotional reflection of their own "shake"
Looks like great fun 
Could you have it wireless and have some kind of tracking so the system still knows where the bottle is so it still projects the image in the right place?
I have a feeling that those wire connections aren't going to last too long being shaken around and pulled etc.
Mowcius
Great demo! One could put some color led inside the bottle as well.
Wow very cool! I like your particle emitter; was this done using processing or openframeworks or something else?
wireless version of such must be great
but how to project exactly over the bottle at free style... is a problem ...
am thinking about it too
particle emitter is done in flash with as3 
kitdashit,
I have the answer to your question ;D: put an infrared led at the top of your bottle, use a camera (regular web camera, strip off IR blocker and put on IR band pass filter, see this http://nuigroup.com/forums/viewthread/2953/
) This way your camera only sees the infrared led and could tell where the bottle is.
You may use the driver on codelaboratories.com for a sony ps3 eye camera and follow one of many online tutorials to mod the camera to only see the infrared led, which is the position of your bottle!) Check out discussion forum on processing.org for how to capture image and process it to find the led position.
I was going to recommend an IR LED but maybe a wii-remote would be simplest. There is lots of info on using it and it is natively supporting at least 4 IR points which would easily allow you to expand the product into multiple bottles for example.
Mowcius
