From some links, it seems that the Rev1 has a few "DNC" (Do Not Connect) pins. However, the diagram on eLinux wiki shows some of them as GND, others as powered (3.3V or 5V). There's also a relevant discussion in raspberrypi.org comment archives, which mentions some Rev1 Schematic (probably this one, see page 2/5, element PH-26/004 in the center), and seems to suggest that on that schematic these pins were actually already defined for Rev1.
So, what's the real status of those pins on a Rev1 board - are they "Do Not Connect", or are they "you can connect"?
So for example, can I take the "Do Not Connect" GND from pin P1-20 to ground some switch connected to e.g. pin P1-18 (GPIO 24) (obviously, with a resistor)? Or, can I use solely the pins 17 (DNC?/3v3), 19 (MOSI), 20 (DNC?/GND), 21 (MISO), 23 (SCLK) to power and connect some external SPI peripheral?