Some friends of mine built a couple of incredible LED-based art pieces for Burning Man this year. They both were based on persistence of vision, like the LED clocks, except that you moved your eyes or head instead of the LEDs moving. They've got a web page here with some great pictures.
They made a 32' tall tower with LEDs down the length of it, in full color, with a Linux box controlling it. You could see it halfway across the city!
Slashdot Mirror
They made a 32' tall tower with LEDs down the length of it, in full color, with a Linux box controlling it. You could see it halfway across the city!