Hi,
I have had the exact same scenario happen to me, moments ago.
We run 2 vcenter instances right now, one is v4.0 and the other one is the new vcenter 5.1.
After we setup vcenter 5.1 we started migrating each host one by one to be managed by this new instance. So we disconnected the hosts from v4.0 and added to v.5.1.
Today I just discovered that some of our Vms were doing some sort of swapping (we had actually hit a vsphere DRS bug). So i manually balanced the hosts in the cluster, but the Vms still showed swappping to occur.
After digging a while I discoverd all our running virtual machines from the migrated hosts had memory limits set. All of them.
Having the old vcenter up and running I went and checked whether they had limits in the old environment, guess what, they had no limits. so it wasn't any setting that translated over.
I looked in the events for a few VMs in vcenter 5.1 logs, none of them have the task/event "reconfigure virtual machine" which is the name of the task when someone changes this manually/scripted. The only event was the event when I removed the limits.
So, I'm not sure your systems administrators are to blame, from the troubleshooting we have done, these limits must have been set when you added the host to the new vCenter. We know this because after moving your 4.x hosts to v5.1 vcenter, we upgraded SOME of the hosts to 5.1. But since we still had 4.x hosts and all our VMs have limits, it is clearly not because of the some bug in the upgraded host.
I hope this helps,