With a Geneva Conventions ID card when coming from overseas. With a driver's license if coming from Mexico or Canada.
I've never travelled out of the country using a passport. Orders and ID.
/johnny
OK, that info is a little old. A drivers license cannot be used to prove citizenship and the US State Department has stated as much. Even “closed loop” trips, such as a cruise to the bahamas or the caribbean, require a birth certificate at a minimum.
Sorry, your information is very dated. ID card and orders for military only.
For civilians, from ALL countries, inc. Mexico and Canada, passport ir REQUIRED. Even for 3 day Carribean cruise starting in Florida or a Sunday bullfight in TJ.