Just as a reminder:

Once upon a time: I compiled a number of filesystems
as modules.
In the installer's initrd, I don't include a /lib/modules, so
/proc/partitions didn't show some of the partitions.
I'm not sure if that was related (fdisk saw them, of course).
But this problem went away when I compiled them back into the Kernel.
It may also have been a bug.

Then again, I don't provide an initrd that loads the modules and I may
as well just include the main components (excluding sound and mouse)
into the Kernel.

** Also rememberto only include what is necessary for the 
   installer to boot: network cards, filesystems and so on.
   Making the Kernel too large results in it not booting **