"I've been meaning to write about PUE, and have been stumped in that It is defined as a metric, and in the Green Grid document referenced it makes no reference that is dynamic. In reality PUE will be a dynamic # that changes as the load changes in a room. How ironic would it be that your best PUE # is when all the servers are running at near capacity, and shutting down servers to save power will [actually] increase your PUE?A broader explanation of Assessing the New Data Center Metrics is on an earlier entry.
If you look at Gartner's description of "Real-Time Infrastructure" (RTI), IDC's "Dynamic IT" or Forrester's "Organic IT" concept, they all point to evolutionary steps where data centers will adapt/respond to demand by re-purposing equipment in real-time. As this happens, the metrics that will define efficiency will need to track this as well.