Here it is not asking for your root password but your user password. This is because you are running sudo, which is designed to allow users to run selected commands as root (or another user) therefore it asks for the user password to verify it is the user running it, and then only if the user has permission to run the requested command (as defined in the sudoers file) will it run the command as root.

I believe the default setting for ssh on the N900 is to enable root login, so why aren't you logging in as root with PuTTY if that is what you want? Otherwise why not keep things simple and just type "root" to get a root shell when you are logged in as a user.