This article is from the FAQ, by with numerous contributions by others.
Permanent resident aliens holding an old Form I-151 Alien Registration
Receipt Card (Green Card) issued before 1979 -- must apply in person
as soon as possible for a new card, Form I-1551 Alien Registration Receipt
Card, to maintain proof of permanent resident status. The old Form I-151
Green Card will not be valid after March 20,1996.
The replacement Green Card, Form I-551 Alien Registration Receipt Card,
will contain the holder's picture, fingerprint and signature. It will
positively identify the holder as a legal permanent resident and make it
easier to obtain employment, social security and other public benefits, and
to reenter the United States from abroad.
The more secure I-551 has an expiration date and will need to be renewed
every ten years. This will allow the INS to progressively improve card
features and make the cards increasingly more difficult to counterfeit.