Don't do this! But, in the file /etc/passwd you'll see a line like:

root:OmfGcryptedpwWTF:0:0:root:/root:/bin/sh

If you change the bit I've put as OmfGcryptedpwWTF to just ! then root is in the same state as user once you save the file - no password is set and login is disabled

Now, the reasons I advise you not to do this are:

1) I haven't tried it
2) I don't know for sure that there wasn't already a default root password set on the device, rather than a ! - I didn't look at mine before setting the password
3) There's a fairly good chance that I don't know what I'm talking about, and doing the above will stop something working.. I'm conversant with Linux, but not Maemo 5
4) Why do it anyway? Just set a secure password and change it regularly if you're paranoid about it..

If you really really want to do it, wait for someone who knows what they're talking about to give you the real information Consider this a placeholder.