According to tweeting travelers, many backscatter and millimeter-wave AIT scanning machines at airports are not in use at all, making opting out impossible. We've asked DHS/TSA for comment, but you can help us confirm.
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Update: The TSA Press Secretary Nicholas Kimball responded to my inquiry—"Was there any sort of TSA-wide policy to not use the machines today or is this an airport-by-airport decision?"—with this statement: "No. Nothing to this at all."
http://gizmodo.com/5698536/fliers-claim-tsa-have-deactivated-body-scanners