See...that makes me think that it's either the VirtualConnects OR the vDS. If you had to reboot EACH blade, then that makes me lean more towards the VC, but what would change between 4.1 and 5 that the VC would act this way.
I agree with you that it's not the Nexus switches caching the mac...that just seems way too far fetched. Out of curiosity, how do you have your active/active links configured because that's the exact same version of VC we're running. Two links in a VPC going to one VC...so 4 links, each pair connected to one vNIC. I started to go to IP Hash, but then I started experiencing weirdness where machines would drop and then come back....but I'll try to figure that one out later, I gotta get this VLAN issue fixed because rebooting each of the blades every time a new VM is created is just out of the question.