I've set up DDNS with dyndns.com server (hostname in dynalias.com domain).
First time it worked, and DDNS record was updated. After I intentionally rebooted my cable modem and DIR-601 itself, DDNS record wasn't updated again.
'Enable Dynamic DNS' checkbox is still checked, all fields seem correct, but 'Status' field shows as 'Disconnected'.
After I rebooted DIR-601 again, 'Status' went to 'Connected' and DDNS was updated. This is the reliability issue, maybe it failed first time since modem wasn't up to speed and connection couldn't be established, and it didn't try to connect again promptly and stayed in 'Disconnected' state.
Please note, that reboot is a common situation, like in case of power outage. IP is likely to change with reboot, and DDNS should always work in this case. Otherwise whole network will become unaccessible from outside due to unknown IP address.