Just because the response from opendns is signed/encrypted does not mean what opendns is giving me is good info.
I think we are now into the academic area. At some point you have to trust someone. Yes, OpenDNS can serve bad data sometimes as bad data can propagate through the system.
A couple of questions: What exactly does DNSSEC do? Does it encrypt the traffic between the DNS and yourself? Or is it merely a way to say "OpenDNS is actually OpenDNS"? If is the latter, then I actually would prefer BOTH - a verification that the DNS actually is the real one, and encrypted traffic so no others can tamper with the data between the DNS and me.
But in both these scenarios are there any way to secure that the data OpenDNS has received is actually good. That is something that will have to rely on the communication they receive. What is important to me, and the only thing I can do anything about, is to ensure that the data gets from OpenDNS to me without going through a man in the middle or in any other way gets tampered with.
The DNS I use will have to take the necessary steps to ensure the data they receive is good. I can only trust that they do it, not do anything about it.