Funny enough, my N800 stopped having boot problems. It was like this:
a) Day 1: Booted several times , no problems.
b) Day 2: Would not boot. Tried again and again, white screen only. Once it helped to remove the battery for a minute, but not the next time. Removed the internal card, it booted.
c) Day 3-> Day N-1: It _always_, without exception, helped to remove the internal SD card (I just re-inserted it after it had booted). Removing the external card made no difference. And it _never_ booted with the internal card in.
d) Day N ->.. It boots every time without having to remove the internal card.

So, what changed? Well, it could just be coincidence, but between a) and b) I made a backup on the internal card (from the n800 backup menu).
Between c) and d) I made _another_ backup, so that there were now 2 backups on the card. (Since then I've made a third one.)
Between b) and c) I also stored other stuff on the card (pdf files etc.) which made no difference.

I think I saw someone from Nokia claim that the N800 doesn't do anything with the card during bootup, and if so the whole thing is totally bewildering, in fact much more worrying. If the boot problem _is_ related to the card then it all makes more sense and the way to fix it should be easier.