The rootfs and other filesystems are still mounted, otherwise your N900 wouldn't boot up, but since it is full mount can't write the information to the /etc/fstab file, though if you are interested you can find the list of mounts kept by the kernel in /proc/mounts.

To solve your problem you need to move something non-essential off of the rootfs, then you'll have space for dpkg to work and you'll be able to uninstall some programs.