p.s. cr0sh, I think the synth shield is a great idea!
I'm full of free ideas; my problem isn't knowledge or money (ok, for some of my ideas, money is the problem) - it's time and other projects that have to come first before I die (not dying anytime soon that I know of - I just have so many damn projects lined up already!).
Yeah - I would love to see a synth shield; it wouldn't be very hard at all to create - like I said, the details are in the coding, but even there it should take much; a few small LUTs for the waveforms (sine, triangle, square), a noise generator (somebody already posted one of those for some Arduino game someone posted on these forums), ASDR envelope programming and control, mixing (or you could do external analog mixing), etc.
This idea is really just a segment of an idea I had to take various parts of things I've seen and build a "gameduino" type system; using multiple ATMega328s tied together via SPI, one of the 128x64 GLCD displays, and an SD card hung off the SPI, you could build one kickin' handheld system (once you hacked the bootloader to download hex files from the SD card).
Unfortunately, I am building my UGV right now - so I leave it for someone else to do (unless I get there first - I can justify it as a hand-held user interface for my UGV...?).