I have had mine 4 days and only had one random reboot yesterday when I tried to play a video and it just rebooted. My N900 had already been slowing down for hours though, I think there is something iffy with qtirreco as its super-slow at times. So I suspect it was caused by something I installed not Nokia.

Granted I also rebooted it several times myself due to telepathy-haze getting stuck with "network error" but you can't blame Nokia for that either.

I sympathise with people who are getting random reboots but its either bad luck being in an area with incompatible cell towers / WiFi routers, or its a hardware fault in the N900. Either way, its a hard nut to crack and I think calling Nokia useless for struggling with it is a little unfair. Yes if I had the problem I would be pissed too but I am sure Nokia are doing their best to fix it and clearly it is something they did not experience in testing.