With how popular the POV toys are, I was thinking an RGB version would get some attention.
I've thought about a 7-segment segment driver. Given what those 7-segment displays cost, I think this way would be more expensive than an LCD shield for more than a few digits, though they do have a certain appeal.
A matrix driver might be a good idea given that the ATmega168 is cheaper than those dedicated driver ICs. With 3 74HC138 decoders, I should be able to handle an RGB matrix off a single Arduino. Though I'd still be using so much IO and processing power I'd want it to be a slave chip to a main Arduino.
I included the stacking headers more to keep it open and expandable, so it's easy to solder in other devices. I'm more thinking I should lose the ICSP header. And I should be able to fit 10 LEDs on there if I wanted.
For what it's worth, I figure a complete kit should cost $20-30, which isn't bad considering the LEDs alone are $2 each at Sparkfun.