I recently acquired another pi and found it wouldn't boot with any of my existing sd cards. It seems there has been a hardware change at some point (new RAM chip?) which requires a new version of the firmware which runs on the Broadcom GPU.
I've updated the dos partition of the sd card image in /contrib/miller with newer firmware files which work for me. If you want to update an already installed 9pi image for a new pi, you can get the files from https://github.com/raspberrypi/firmware/tree/master/boot But at the moment, it's best not to use the very latest firmware, because a change was made on 18 September which breaks usb quite badly for Plan 9. [See .../raspberrypi/firmware/issues/214 for details if you're curious.] I hope this will be sorted out eventually, but meanwhile you can follow the History link on the above page and use a version earlier than this date.
