Read this - quite interesting...
On
www.jkrowling.com in the Rumours section.
Question: When the Marauder's Map is insulting Snape, how did Prongs write his insult he's dead?
Answer: Wizards have ways of making sure their voices are heard after their death - think of Bertha Jorkins rising out of the Pensieve in Goblet of Fire, The Sorting Hat continuing to Spout words of wisdom of the Founders hundreds of years after their deaths, the ghosts walking about Hogwarts, the portraits of dead headmasters and mistresses in Dumbledore's office, not to mention Mrs Black's in number twelve Grimauld Place...there are other examples too, of which the Marauder's Map is merely one. It is not really Prongs writing the insult to Snape, it is as though he left a magical recording of his voice within the map
How interesting....I'm intrigued and can't wait to see what she's got in store for us. I think she means that people who have died may reappear somehow.