Hi...
According to Post, vSphere 5.0 remebers the last state of VMs, so in case of entire-cluster Power outage, vSphere will start VMs accordingly to their last state.
Need to be verified in the Lab...
Check this too.. verified by one of the masters of HA in the world, Duncan Epping..
http://www.yellow-bricks.com/vmware-high-availability-deepdiv/
Check the (All Hosts Down Section)