Have a look at this customizable Badge with scrolling LED message from ThinkGeek.
The gadget is completely programmable to be able to display what you want. You can change the message easily using three control buttons and it’s possible to load up to 6 different messages into the badge.
You can even set the scroll speed and brightness level and it’s smaller than a credit card. The battery life is about 18 hours.
This said, any idea about what could it be used for? 
