Besides the touchscreen, I wish to use buttons to control a few other components of the system (such as the intercom buttons) and maybe a numeric keypad. The obvious (and easy) solution is to use standard buttons. There is of course several options and variety for those. But I was also curious how other devices (such as some of the IPODs and even laptop mousepads worked.
After a little research, most of these devices uses a form of capacitive sensing to detect contact (or proximity) of a grounded object such as a finger. Below is a little disgram which illustrates how they work.
The finger (or other grounded material) essentially drains up some of the field between the two electrodes, which is detected by the chip and considered as a button press once the change reaches a certain level. This diagram is taken from the documentation of the Analog Digital AD7142, which is a 14 channel capacitive sensor chip.
The AD7142, has 14 channels which allows for the implementation of over 36 buttons, more than what I need for my application... In large quantities, the chip goes for under $2.00 per unit, which is a good price. The main drawback for me is that it is a surface mount component (but then again most chips are these days) but for now I have ordered two sample units from Analog Devices so it will give me something to tinker with. So I will have to start learning and experimenting about homebrew surface mount techniques.
Another design consideration here is that there must not be an air gap between the sensor PCB board and the device cover as air has a tendancy to dissipate the field and therefore reduces the sensor's sensitivity.