Yes, a DynDNS domain will resolve into an IP, but an IP won't correctly resolve to a DynDNS domain unless you specifically ask DynDNS. When I use the same site,
http://www.whatismyip.com/, to do a reverse lookup on my IP, I get a RoadRunner domain name, not my DynDNS domain.
Now, the question is what's passed to OpenDNS? I know the IP address is passed - that's how you get your reply. But is the Domain passed? Not unless you fill in the DNS Suffix field. The problem there is that my LinkSys router doesn't have any place for me to enter that in. So I can't pass that to all the clients via DHCP. So I'd have to manually enter that for all devices on my network - a major pain.
Doing some poking around, I can see where this is an issue.
This link makes it clear that content filtering is based on the public IP.
This makes clear the non-relationship between OpenDNS and DynDNS.
So in practical terms for the home users, there seems to be two choices: Use a static IP address or run OpenDNS program to keep OpenDNS informed on your current public IP address.
Hmmmmm, I'm seeing things that suggest the "tomato" firmware may support OpenDNS - among other features. Since I have a older Linksys, that may well solve things for me.