http://www.cnn.com/2006/US/08/15/Ter....ap/index.html

The government's new order that all airline passengers put their shoes through X-ray machines won't help screeners find a liquid or gel that can be used as a bomb.

....

In its April 2005 report, "Systems Engineering Study of Civil Aviation Security -- Phase I," the Homeland Security Department concluded that images on X-ray machines don't provide the information necessary to detect explosives.