These guys did a killer demo at NYTM. Very cool demo of turning a plant into an instrument (they played an orchid on stage). There was a good question about detection of differing touches of the doorknob, this could totally replace the key some day!
I'm reminded of #66 on the Evil Overlord list - My security keypad will actually be a fingerprint scanner. Anyone who watches someone press a sequence of buttons or dusts the pad for fingerprints then subsequently tries to enter by repeating that sequence will trigger the alarm system.
Ah, sign of the times. This list was compiled in 1996.
As this approach uses only a single electrode (which is why it is so cool), the data it offers is quite limited compared to more complex sensor layouts.
In real life, such a sensor would need to work reliably even if I wear different shoes, have dirty hands, etc. Therefore, its false rejection rate needs to be very low. Which leads to a high false acceptance rate, i.e. several other people with similar capacitive footprints may open my door, too.
[1] http://chrisharrison.net/projects/capacitivefingerprinting/C...