From
http://www.tsa.gov/public/interapp/e...image_0083.xml

Coat Policy

TSA’s new protocols also require all passengers to remove outer coats and jackets for X-ray before proceeding through the metal detectors. Included are suit and sport coats, athletic warm-up jackets and blazers. However, if a sport coat or blazer is being worn as the outermost garment – not being worn over a blouse or sweater, for example – it does not have to be removed.

Sweaters and sweatshirts may remain on unless they contain a significant amount of metal or their size and bulk could potentially be used to conceal dangerous articles.