I've gotten these letters. I'm sure you've gotten them too, or something very, very similar. They are crafty little pieces of devious work. There's a really interesting run-down of some of the worst junk mail offenses commited by these "official" looking missives over on Evil Mad Scientist. I liked seeing the collection and knowing what to watch for in the future. Of course, they aren't totally useless; you can always turn your junk mail into mail art!
(Found via BoingBoing, a keeper of my company for many, many Internet hours)