A button you don't push so much as touch because it's got a capacitive touch sensor underneath. It means that random touches suddenly become meaningful.
Playing with a friend's Android (probably the Omnia 7 mentioned in the linked article) I'm finding the capacitive buttons on the bottom (the standard Android four buttons) really annoying because once they're active, they're all active and you can brush one meaning to touch the other.
Just to clarify, I believe mkr-hn means the trackpad itself, not the buttons on the trackpad. Most trackpad buttons these days are physical. The trackpad itself, however, is capacitive.
