It is random and can be quite occasional. Hasn't happened since I last posted and it was some months between crashes before that, until I started jumbling around the hardware.
I have a multi-boot box, using Mandriva Linux x86_64 99% of the time with nvidia proprietary driver 295.40 and 32-bit XP-Pro SP3 up-to-date used very infrequently. Now that you mention it, this might have occurred on a boot to XP but I don't recall. I don't cold-boot to XP, only reboots from Linux, and, AFAICR, this only happens on cold boot.
Google gtx 470 random reboot
and there seem to be enough reports to implicate something about the card, itself. IDK but re-installing drivers or driver cleaners does not seem to be so much the way to solve problems in Linux. I can imagine nVidia writing the same flaw cross-platform to Windows and Linux drivers but I really don't know enough to draw conclusions.
ATM, not happening for two months, it's more like an annoyance and curiosity, unless it gets more frequent. There might be other elements already written to this thread. Looking at the X log is a good idea and I'll try to remember to do that if/when it happens again. Maybe the card has settled down after the recent musical chairs of parts.