@sakya

this can be fixed by multiple ways
but as long as i don't have your N900
the best way i can tell you is :

1-) install that version which dpkg is requesting
2-) download the portrait version from the first post
3-) dpkg -L hildon-desktop # then remove the things shown
4-) dpkg -x hildon-desktop-portrait / # as root

PS:
if you wanna remove it don't just install new hildon-desktop version
you need to install the version from first post with dpkg -i
then remove it

LAST NOTE: haven't tried this on my N900
but there is no reason that it will not work
although this is at your own risk