I have never had success with 'sudo gainroot'. I always just type 'root' to become root and it does not ask me for a password. YMMV.

I was about to ask the same question about the default user password for SSH. Running 'passwd' as user gave me an error ("The password for user cannot be changed"), but having found this thread pointed me in the right direction. As root, run:
It will prompt you for a new password, not asking for the old one. It said min 5 max 8 characters but I used 4 and it did not complain. The next time I did ssh user@<my N900's ip>, I used the password set as above and... voilá!