You cant actually use a MAC address which is assigned by vCenter as a static MAC address, there is a different range set aside for the manual mac addresses which must start with 00:50:56, more information on this can be found here: http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=219
Yes, PowerCLI (and PowerShell) -- powerful indeed, as the names imply.
Setting the MAC address for a NIC is mostly straightforward. We recently wrote about it a bit ago at vNugglets.com. See the Setting MAC Address for VM NICs using PowerShell post from a couple of months ago.
The code there will set the MAC address type to static and set the MAC address to the desired value. This handles updating the VMX -- no further interaction should be needed.
And, btw, if you wanted to get the VM's current MAC address first, you can use:
Get-VM myVM | Get-NetworkAdapter