DynDNS now expires free accounts if there has not been a update in the last 30 days. I just had to recreate my account because it expired. There really should be a update interval regardless of ip change to prevent expirations from happening.
https://www.dyndns.com/services/dns/dyndns/readme.html#inactivityAlso a thread here asking about update intervals but unrelated to account expirations.
http://forum.m0n0.ch/index.php/topic,2632.0.html