I have an OpenWrt 10.03, and I need to update my dynamic DNS account (e.g., OpenDNS or DYNDNS) when my IP changes. How can I do this from the terminal?
You could look at using inadyn, a client to update your DNS entries.
There is a page about it on the DynDNS support site.
Alternatively, there is this line:
curl -v -k -u user:password "https://members.dyndns.org/nic/update?hostname=&myip=$(curl -s http://checkip.dyndns.org | sed 's/[a-zA-Z/ :]//g')&wildcard=NOCHG&mx=NOCHG&backmx=NOCHG"
 Untested: from commandlinefu: http://www.commandlinefu.com/commands/view/2492/update-dyndns.org-with-your-external-ip.
If you have a recent version of openwrt and you have a package called luci-app-ddns where you can configure it on your web management console. Also check the OpenWrt documentation on this issue.