Most of us are familiar with the standard LED message signs that use a fairly large 2 dimensional array of LED's to communicate information.
Many of us are probably familiar with the "persistence of vision" clocks which use a small 1 dimensional array of LED's to display time. The second dimension
is created by moving the array and letting the human eye and brain "think" that they're seeing a 2 dimensional display.
Now to the point: Add a one dimensional array of LED's down the side or in the keyboard or on the back or wherever on a cellular phone to create a persistence-of-vision messaging system. The necessary motion would be supplied by the user holding the phone and waving it side to side in the air in a 3 or 4 foot wide arc.
The speed and direction of motion would be detected by one or more accelerometers in the phone. The programming would be done via the telephone keypad as is done for SMS text messages.
An LED, visible to the user and separate from the array, could flash at an appropriate rate to provide pacing cues to the user/waver for optimal display. The LED pacing flashes would represent the points in time that would optimally coincide with the maximum left and right positions of the phone in the air. A visual metronome sort of thing.
Optional would be a method for attaching a stick for waving the phone up higher in the air.
(Note: this has been posted before, under a now deleted account, for the purpose of yet again proving that ideas are not summarily rejected when the author is not a longstanding halfbaker.)