PCB Matrix Touchpad?
I've done exactly that for this product:  http://www.qscaudio.com/products/network/QSys/pagestation.php

Behind the lexan self-adhesive label is a 2 layer PCB using a Cypress Semiconductor PSoC1 chip.  The chip is great, but their software is terrible.  You'd probably be much better off using their PSoC3 series, or using one of many other MCU's that has cap-touch support.

Cypress has lots of app notes on how to do your PCB layout for cap-touch.  We did descrete keys, but their app notes also cover sliders and track-pad type stuff.