Sorry MissBlue I didnt see your post when I was answering
But yes, when we had our wedding last year we had a lot of guests who were getting there first passport and so the card vs. book question came up and the Clerk Of Court here told me it could be used on cruises to the caribbean but not flights

I just clicked on your link above and it says this:
The passport card is the wallet-size travel document that can only be used to re-enter the United States at land border-crossings and sea ports-of-entry from Canada, Mexico, the Caribbean, and Bermuda. The card provides a less expensive, smaller, and convenient alternative to the passport book for those who travel frequently to these destinations by land or by sea.