This article is from the FAQ, by with numerous contributions by others.
For applicants with a valid, unexpired passport: Applicants may bring
their passport with evidence of filing for a new Green Card, or evidence of
filing for naturalization, to their local INS office. A temporary Form I-
551 stamp, valid up to one year, will be placed in the passport.
For applicants without a valid, unexpired passport: If there is a need for
emergency travel, applicants may bring evidence of filing for a new Green
Card, or evidence of filing for naturalization, to their local INS office
and request a temporary Form I-551. If the request is clearly warranted
because of an emergency, the temporary Form I-551 will be issued.
These authorization documents allow the applicant to reenter the United
States and obtain entitlement benefits until he/she receives a replacement
card or becomes a naturalized citizen (if the applicant applied for
citizenship instead of a new Green Card.)