michaaa62 just pointed me to a possible similar case of a "read-only" flash MMC problem.
http://nourgomaa.com/my-nokia-n900-infected-chip/

It warns not to flash the device too often and not use backupmenu etc.
This seems very strange to me. The only thing what restoring an image in backupmenu does is formatting a flash partition and copying files. I don't see how that could cause any kind of physical damage.
And why a -R option in flasher would have any influence is also beyond me (it just tells the N900 to reset).

But I will open up the N900 if the relocation/badblock tagging doesn't work and find out if my N900 too has any kind of damage.