OK, supper was good… ;)
and to the point now, I did look at my setup, and to my surprise, I'm not actually using the DDNS capabilities that are built in to Tomato… I guess I'm doing my own "home brew recipe"…
1. My DSL router is set up w/ port forwarding: external (WAN side) port 80 gets mapped to internal (LAN side) port 81 on my WL-500gP v2, which is running thttpd, for my weather service (see my sig, http://davysweather.dyndns.org/ ).
2. I have inadyn running on my router, via Optware. It checks the external IP every 20 minutes and keeps my account active/current.
I'm guessing that #2 from above can be done via the DDNS page you are using, but I just didn't do it that way.
AFAIK, it is essential that you use some sort of port forwarding on the sort of DSL/Cable router that most homes/small businesses use.
hth, apologies (pre-emptive ones at that) for any mistakes or security no-no's that folks w/ more knowledge (perceived or real) may find in my comments.