We're also having the same issue I am upgrading one host at a time from 4.1 to 5.1 on vCenter 5.1 and it has happeed on both ESXi versions. I log into the host directly using the VMware client or console using esxtop and they both show the host using resources. This is a big problem for me because when it show zero resources DRS starts to migrate all the VMs off to other hosts. Last week it happed on 4 host and I started to see performance issues because all the VMs were vMotions to the other hosts. Instead of reseting management it is easier for me to just disconnect and reconnect the host from vCenter.
VMware Please fix this.