Thanks for the hint! I couldn't find anywhere to set the node details in NCM directly, BUT it did show me something I missed.. when looking at a node in NCM the SNMPv3 settings show the correct username and authentication type.
The thing I noticed is that the 'Encryption Type' shows as none. I searched the forums here more and found this:
"Submitted a TAC. Answer is NCM does not support SNMP v3 AES-192 or AES-256 encryption. Planned for future release."
http://thwack.solarwinds.com/thread/55380
I'm going to validate that is still the case as well.. but that looks like exactly the problem I'm having.