Assuming now that you have done nothing on the system except copy your dcerouter's Sambacredentials file to the MDs, it seems to me there is some bigger problem with your sambahelper account and I'm afraid I don't know how to fix that if it is the case. As alluded to so delicately by Thom... the sambahelper account is created automagically and should not be messed with.
I asked a friend to check his working 10.04 installation and he confirmed the following:
1. sambahelper exists in /etc/passwd on dcerouter as expected:
grep -i samba /etc/passwd
#> sambahelper:x:10002:10002:Pluto Samba Share Helper:/tmp:/bin/false
2. He does not get an error when he lists the users in the smbpasswd databse
sudo pdbedit -L -v | grep -i username
Unix username: pluto_mybuddy
Unix username: pluto_mybuddyswife
Unix username: sambahelper
3. His dcerouter version of Sambacredentials.secret differs from his single MD's version by password:
diff /usr/pluto/var/sambaCredentials.secret /usr/pluto/diskless/XX/usr/pluto/var/sambaCredentials.secret
(Obviously I changed the MD number to XX and the hashed password values)
So I'm guessing that the dcerouter Sambacredentials.secret file isn't an issue for him because the mediadirector there is not trying to mount a remote share and that the credentials on the MD are correct (ie the password there, when hashed, matches the smbpasswd db).
So, I may be wrong on any of my assumptions there but, right or wrong unfortunately in your case I think the issue is the lack of the sambahelper user acc. Not sure how that happened but maybe one of the gurus can give some guidance there? Otherwise shoot down my assumptions!
davegravy... might be interesting to get the results from your setup with the commands above?