Hi D'Arcy,
Depending on your internet connection method, sometimes, other factors can prevent SME from detecting an IP change and therefore not update your IP address with DynDNS.
For example, using the bpalogin add-on for SME to handle cable logins with bigpond, occasionally, when Telstra goes off the air then comes back on, SME does not detect the change in IP and therefore does not trigger the update ip event. In the end, I gave up on the built in mechanism and installed ddclient then set ddclient to check my IP address from the web instead of getting it from eth1 (the external I/F). This appears to have worked out better. One added advantage of using ddclient is that you can update any domain name (your SME server does not need to be set to the domain name you are trying to update -- although this you can modify templates to workaround this).
Hope this helps.
Kelvin