Many thanks for all the replies.
I am aware of the defaults and alternative functions of all 54 BCM2835 GPIO pins, and I know which pins are availabe on the GPIO header(s) for the various versions of the RPi. What I don't know for certain is what the RPi uses the other pins for.
According to http://elinux.org/RPi_Low-level_peripherals
GPIO16 drives status LED D5 (usually SD card access indicator)
GPIO28-31 are used by the board ID and are connected to resistors R3 to R10 (only on Rev1.0 boards).
GPIO40 and 45 are used by analogue audio and support PWM. They connect to the analogue audio circuitry via R21 and R27 respectively.
GPIO46 is HDMI hotplug detect (goes to pin 6 of IC1).
GPIO47 to 53 are used by the SD card interface. GPIO47 is connected to the SD card interface card detect switch; GPIO48 to 53 are connected to the SD card interface via resistors R45 to R50.
But what about the rest?