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.
Not every airport in the US even has the "Advanced Imaging Technology" scanners installed. (A post at FlyerTalk.com has an up-to-date list of airports with the machines, as well as specific terminals.)
Gizmodo commenter "The Dewd" posted the above image showing Twitter updates that indicate that many of the machines are off, including this tweet from "_msw":
Through TSA checkpoint at SEA in 10 minutes flat. Many backscatter scanners are off. No scan, no patdown. #SEA #optout #TSA
A report from the Associated Press, however, includes an image showing a woman in a body scanner at Seattle's SeaTac Airport dated today:
An airline passenger raises her arms as instructed in TSA's new full body scanner at SeaTac Airport Wednesday morning, November 24, 2010. The crowd level was normal, and there were no delays. Peter Haley / Staff photographer
The presumption by some is that the TSA has deactivated the AIT machines in an effort to both increase throughput of travelers through the scanning process and to diffuse protest by the grassroots Nation Out-Out Day movement.
But as the AP photo shows, it's not always possible for a single passenger to determine if an airport is using the machines at every terminal or not.
We will update this story as we get more information. If you are flying today, it might be useful to simply ask the TSA officers doing the screening if they are using their AIT machines at all.
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."