I'm almost done with my new project. I only need a better video explaining everything, but I couldn't wait any longer posting about it.
CastDuino gives you an Arduino (master) that can send 8 text messages (80 characters for now) and 16 action messages to other Arduino's (slaves), which will show the messages on a screen of your own choice or/and can do an action predefined by yourself. All this can be done through the Master Arduino's embedded web interface.
I'm not able to do that right now, but I managed to get TV-out working with nice scrolling, so here is my test video showing you a you internet time and a message that has been broadcasted by a CastDuino Master and stored by this CastDuino Slave in it's EEPROM. The message is time dependent and only shown within the timeslot, configured in the Master and broadcasted along with the message.