When an ESX/ESXi host has been added to vCenter and the vpxa agent has been installed on the host, the vpxa agent will not allow any virtual machines state changes to be made directly from the host itself. You will still be able to use the vSphere client to connect directly to the ESX/ESXi host, but you will not be permitted to power on VMs directly on the ESX host. You will also be denied access to power on VM’s using the CLI. Only through vCenter will you now be able to power on a VM. What makes this awkward is this: The error message that is returned when the vpxa agent denies power on operations does not state the reason why your VM has failed to power on.
I got caught out by this after my vCenter Server (that was running as a VM) was powered down. I then connected directly to the ESX host where the vCenter VM was registered, but I was unable to power on the vCenter VM as vpxa was denying the power on operation. The way to get around this is to stop the vmware-vpxa daemon on the host “service vmware-vpxa stop). Only then will you be able to power on VMs directly.
I also have to state that I have not tried the GA build of vSphere 4.1, so the only experience I have with vSphere 4.1 is the beta. It would be interesting to see if it's still the same. Feel free to test it out and leave comments.