I am currently using the DDNS setting in my DNS-323 with the latest beta. It appears that it's properly updating the account and not allowing it to suspend. Other equipment, including several different D-Link routers, used to let the account expire.
I think if you upgrade to 1.08b8, the DDNS will work correctly.
BTW, I have a DynDNS account, which is the same service you would be using if you registered for D-Link's DDNS. I just used my name/password and all is well.