silvrlvr
Refugee
if you can get there at about 3:30-4:00 in the afternoon, they hand out numbers for lining up. you can get tickets at that time sometimes. i grew up in ct, and used to go to letterman frequently. i have done the go over at 3 many times, without having tickets and gotten in. if you are close by its worth trying.
Also, sign up on his Web site for the Late Show Newsletter. Every week, they take requests from people who are on the mailing list for tickets. You can tell them when you'd like to go.