Wouldn't it be cool to have animated and programmable tattoos? You could show off your latest Flash animations, watch TV on your arm, have a built-in PDA screen.. The possibilities are endless.
Here's how you might do it. Develop very small spheres that can change color or just turn on and off
(super-efficient LED's?) and inject them just under the skin. You'd need a pretty large array of them even if you want minimal resolution (200x200) so we're probably talking on the order of a 0.1mm diameter sphere.
You'd need to have (1) some way to store or obtain power, (2) light generation facilities, (3) communication, and (4) logic.
(1) since the device is very near the skin, there's usually a temperature gradient between air and body temperatures. There are very simple thermoelectric devices that are now used for power generation or refrigeration - they aren't so efficient yet but maybe we could get by with a very small amount of power.
(2) miniscule LED's? I guess you'd have an array of red, green, and blue devices like a CRT.
(3) you'd have some kind of controller in your pocket that would communicate with the LED's at some frequency. Two problems: we're talking a 0.1mm antenna - IIRC the antenna size has to be of the same order as the wavelength, so we're talking infrared or visible light. Not so convenient for this application. However I think there are things called fractal antennas that could fit in a smaller package, that might get you down to the microwave region. Second, it won't be long before we have jamming devices that will let you hack into somebody else's tattoo and make it misbehave ...
(4) that's the easy part - you'd make the whole thing on a chip in the first place (so the electronics is no problem), then detach the devices using etching chemicals and coat them with plastic or something non-bioreactive...