I should state that I have Vsphere 4.0 Essentials Plus, so I don't have Vmotion as an option.
I have two hosts, and one of them is running Win2008 as a guest with vCenter on it managing my two hosts. HA is enabled. I have vCenter running on "HOST01." When I powered down HOST01 "HOST02" booted up my vCenter VM as planned. Once I bring HOST01 back online, how do I migrate the vCenter VM back to it without rebooting HOST02? Of course I can't Vmotion it back, and if I turn off my vCenter VM, I can't migrate it through vCenter (ha!).
Power off VC VM.
Then browse the datastore from the second ESX, enter in VC folder, right click on *.vmx file and add to inventory.
Power on the VC VM and specify that you have moved the file (if you will see a question after the power-on).