I just spotted this 'client key' option for updating domains. It seems like this is a different password/key for updating your domains from the password you use to log into your dyndns account (which has been a long time gripe for many users). I have not generated a key to try this yet, since this has not been implemented yet in the tomato and I don't know if I can go back to not using a key after it has been generated. This is all of the info that they posted in relation to the option:...
New option to update DynDNS domains with 'client key'
New option to update DynDNS domains with 'client key'