Bart is yet again correct and this is from the tsa website (which i carry with me). scroll down and bolding is not mine but on the website:
http://www.tsa.gov/travelers/airtrav...eds/index.shtm
People with disabilities, those with prosthetic devices, and those with medical conditions
do not have to remove their shoes during this heightened security level nor has this been the case during routine travel times. Those who keep their shoes on will be subjected to additional screening that includes a visual or physical and explosive trace detection sampling of their footwear.