Originally Posted by Varius

This is the latest info I could find:

ENTRY AND EXIT REQUIREMENTS: On December 31, 2005, the U.S. Government will begin to phase in new passport requirements for U.S. citizens traveling in the Western Hemisphere. By December 31, 2007, all U.S. citizens will be expected to depart and enter the United States on a valid passport or other authorized document establishing identity and U.S. citizenship. The Department of State strongly encourages travelers to obtain passports well in advance of any planned travel. Routine passport applications by mail take up to six weeks to be issued. For further information, go to the State Department's Consular website: http://travel.state.gov/travel/cbpmc/cbpmc_2223.html.

For entry into the country, Costa Rican authorities require that U.S. citizens present valid passports that will not expire for at least thirty days after arrival

So I would say yes, thy probably do require it....however they are usually so lax at the airport here that you could probably show a library card and get by