Back to it: I breadboarded that voltage-divider keypad and had an Aargh! moment.
Thanks to real small spread between keys on the lowest row (*, 0, #, D), key press resistances bobbing around from 25 Ohms to 45 Ohms, and 5% resistors, my "0" and "#" keys had identical values. I triple-checked the wiring, and found a mistake, but it still had the same problem. So I have to rework the resistor values to get larger ADC increments between keys and try, try again.
But the basic idea still holds true. You can have multiple switches (read: keypad) connected to a single ADC pin and figure when one was pressed and get a value that determines which was pressed.
Later!