Here's a quick rundown of the rules as they stand today:

Passengers traveling by air: Now, all U.S. citizens (including children!) must present a passport or another approved type of secure travel document when they enter any U.S. gateway via air. Don’t worry about the “secure travel document” comment, unless your clients serve in the military or meet other special conditions; the basic rule now is presenting a passport when clients return to the U.S.A. via air.

Passengers traveling by land or sea: Now through May 31, 2009, U.S. citizens ages 19 and older must present a valid passport or another combination of approved documentation establishing both (a) identity and (b) citizenship. Meanwhile, U.S. citizens agents 18 and under only need to show proof of citizenship, such as an official copy of their birth certificate.

Starting June 1, 2009, though, these land and sea rules change. On that date, the requirements for re-entering the U.S.A. via land or sea will convert to the air standards above - a valid passport, basically. However, your clients can also opt for a wider range of WHTI-compliant documents that will also work for land and sea travel. For example, the U.S. government is already taking applications for the new passport card that's less expensive than a full-blown passport, but they only work for trips within the Western Hemisphere.  Also, some states like Washington have begun issuing “enhanced driver’s licenses” with security features that make them acceptable as ID to federal officials. (Other states working on such licenses including Arizona, New York, and Vermont, but more states will follow suit.)

Note that one new option for land and sea travel - getting a U.S. passport card -- will only work for land and sea travel. Passports cards will not be accepted for air travel.