I could probably use the drawdio's signal to detect presence but I'd like to communicate some data. I did some more research last night and it looks like there are two companies 3 years ago that announced chips that were like rfid's but instead of proximity it was touch oriented. I unfortunately can't prove that they ever released any products. One was NTT's RedTacton and the other was ...I can't find it now. If I could get a part number I'd hit up mouser but I can't even find their companies listed.
The FSK seems like the right idea, it seems to be used primarily for radio (FM being a form of it) but that's ok, a weak enough signal and you have to touch the receiver for it to pick up a transmission. =) I have no idea what kind of voltage i'd need or how the properties of skin affect it or even any chips to support encoding decoding, but I'm starting my research. (I've got about 8 different wikipedia and google tabs open)
I was wondering if I could just use pulse width modulation instead as that (producing and reading pwm) is built in to the audrino. According to the docs analogWrite() producing a pwm with a freq of about 490 Hz, a far cry from 100khz. Maybe there's an easy way to encode/decode that pwm on a 100khz carrier? Or am I getting ahead of myself and ignoring an easy uart/serial, shift register or parallel (hopefully ttl) chip that does the FSK?