The police are treating this seriously. Here is an excerpt from the Guardian, dated 2 hrs ago:

"Officers in Portugal are investigating the letter's claims and are poised to visit Odiaxere. Dutch police said part of the reason the letter was being taken so seriously was because it was similar to one sent to the same newspaper last June following the disappearance from the city of Liege, in Belgium, of two girls: Stacy Lemmens, seven, and Natalie Mahy, 10.

In that case the author claimed to know where the girls had been buried. The children's bodies were found two weeks later in a storm drain near a railway crossing a short distance from the location identified in the letter."

Prayers for Madeleine and her parents.