The passport card is only valid for travel via land or sea, therefore only valid for travel to Jamaica and the Caribbean when arriving by ship. It is NOT, as far as I can tell, valid to be used when arriving by airplane in Jamaica. I do hope they change that some day, but as of now it is not valid.
Just thought I should add that note so nobody gets screwed up. Furthermore, if you already have a passport they are asking that you send your passport in to them with the application for a passport card. This could leave a traveler without a passport for a while and could possibly prevent them from traveling abroad until their passport is returned to them.
Irie