I reported way back that v1.15 has an issue with enabling Dynamic DNS.  I just upgraded to 1.21NA this morning, and it is still broken.  When examining the logs, I see the following:
[WARN]   Fri Aug 28 09:52:17 2009   Error updating dynamic DNS entry: www.[domain].net badauth. Please check configuration. Disabling DynDNS
I used both the Authkey that DynDNS created for my domain, and I even tried the actual user account I use to authenticate to the admin portal on DynDNS.  Both times I recieved the "badauth" message above.  What needs to be done to fix this?