So as there is a thread here
about how to do it from windows, I'll post my method on how to do it from linux.
As I didn't want to interfere with the user "user", I created a new user, called free. You will need root access for creating new users.
I also have not bash installed so the new user should be assigned the shell /bin/sh.

On the n800:
1) start xterm
2) 3) Fill informations.

On the linux:
1) If not already done, create your personnal keys (your computer/your login).
By default it will create an RSA key for ssh v2
2) ssh-copy-id free@ip.of.the.n800 (or if you don't have ssh-copy-id, manually copy the content of id_rsa.pub to /home/free/.ssh/authorized_keys on the n800 and set the rights to -rwx------ for .ssh directory and -rw------- for authorized_keys, both being owned by free:free )
3) If you don't want to be asked everytime your secret key, use ssh-agent (it will deal with the keys and give them to the remote ssh-server when logging to the remote host). It should be started by most session managers (kdm/gdm at least) in linux (If not, start it manually and check that SSH_AUTH_SOCK variable is set and exported)
This has to be done everytime you log in to the linux.
Give your private key
4)Connect and it should'nt ask you your local private key nor your remote password.
Now for security reasons, you can disable the password of the user free so that nobody can remotely brute force your password. Only your linux user from this linux and which gives the correct passphrase to ssh-add will be able to connect.
On the n800:
usermod -L free

That's it!

edit:
doing usermod -L free locks also the public/private key mechanism. The rest is fine. More info when I understand what's going on..