Do you have none of the applications installed to gain root rights, like openssh, rootsh or sudser???
To get some information about disk space try the following code in the X-Terminal applicationEach of the lines is one command, the last one a real challenge for linux newcomers because it combines two commands with a 'pipe' symbol '|', this no small 'L' or large 'i' ! You may access the symbol on your N900 by typing first the blue arrow and then 'Sym'.

Try to gain some space back to install rootsh by disabling (removing) some applets from your desktops, try to remove some applications through the Hildon Application Manager (HAM), disable any repository in HAM but extras, it will refresh the list, then close HAM and reopen it. Try to install rootsh.