Looks like Letterman is retiring next year. Makes sense, and makes the Fallon-in-NYC decision look even smarter if CBS is going to have to figure out how/where to replace Dave.
ETA (via gothamist): Allegedly, this is Letterman's statement from tonight's show:
The man who owns this network, Leslie Moonves, he and I have had a relationship for years and years and years, and we have had this conversation in the past, and we agreed that we would work together on this circumstance and the timing of this circumstance. And I phoned him just before the program, and I said 'Leslie, it's been great, you've been great, and the network has been great, but I'm retiring.'
I just want to reiterate my thanks for the support from the network, all of the people who have worked here, all of the people in the theater, all the people on the staff, everybody at home, thank you very much. What this means now, is that Paul and I can be married.
We don't have the timetable for this precisely down - I think it will be at least a year or so, but sometime in the not too distant future, 2015 for the love of God, in fact, Paul and I will be wrapping things up.