Hello-
I'm actually trying to gather up a design for a DIY digital piano. I'd like to use an Arduino as well. If you are building a keyboard with only a few octaves it should be relatively easy, but for an 88 key keyboard it looks like I'm going to have to use several external IC's.
I think the circuit will be something like:
2 Arduino outputs -> 4 bit Binary Counter -> 4 to 16 Decoder (really a couple 3 by 8's ) -> 16 x 11 diode matrix -> 11 Arduino inputs.
That assumes I can use the RX pin as either a digital in or a digital out, while still using the serial port on TX.
It's a work in progress, but you can check out diykeyboard.org
Drew