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?

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I can certainly say that DNC2 and DNC3 are Ground and DNC1 if +5v, I have used those pins in previous projects and they have behaved as I would expect them to.

My guess is that the DNC pins were reserved for the additional GPIOs which are present on the Rev2 board, but since they were used for ground and power before then the additional 8 pin header on Rev2 boards had to be added (this is quite possibly wrong, but seems to be logical).

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