@zoomkat: Have you looked at the prices of illuminated pushbutton switches on Digikey/Mouser? If I wanted to spend $300+, I would just buy a Monome kit.
@MikMo: Yeah, I'll probably end up doing that. I don't have enough cash right now to order everything I would need anyway. I'm just going to pick up some stuff for my parallel EEPROM reader/writer and a couple of components here and there to mess around and prototype with in the meanwhile. Then maybe I can save up enough to do a 256 LED matrix.
Also, I have decided how I am going to construct the LED buttons. My GF actually came up with this. As an artist, she's amazing at finding weird, awesome crap to re-purpose.
You know those capsules that cheap vending machine toys come in? The small ones? You can buy those in bulk for super cheap. (1000 for $40, for instance)
Sandpaper the capsule to diffuse the light from the LED, glue the base of the capsule on permanently, drill a hole in the base so that the LED can fit through, and mount a momentary switch on the side to be pressed down by the base of the capsule.
Then all that's left is to build a plywood/acrylic cover for the capsule button tops to stick out of.