I just recently got the same letter. If you go to the old forum and search for air marshal, you should find previous posts on this subject. If I remember correctly it stated they have all or most of the applicants they want. Prior security, law enforcement, or military was a major plus. From what I understand the letter implies that you are elligable to move along in the process if selected. No info on the interview, sorry.
It might make you feel better to know that they certainly do send out letters indicating ineligibility, as I got one, so Congrats! That was indicated on the report they sent, which was the first piece of correspondence I received after applying.