Well you could check the logs on the ESX hosts that are up: /var/log/vmware/vpx/vpxa*.log. You will see details when a VM has been powered on, something like: powerOn on vim.VirtualMachine:64

64 is the ID of the VirtualMachine. You could resolve that with some scripting to get the VirtualMachine name. Seems like a lot of work to me, but using the perl toolkit the process would be something like: read in the AAM, build an array of the ID for the powered on VM's, connect to VC and then resolve the ID to the friendly\displayname that you see in the inventory pane in the VI Client.

Would be a good scripting project, but I haven't got time at the moment

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If you're writing an sdk script you can look for the VmRestartedOnAlternateHostEvent events.