11:30pm ET/PT on CBS (check your local listings)
Yep!
These kind of things are always so set up, it benefits them both, it sets up all these arguements and gets everyone talking, everyone will be watching O'Reilly's next show to see what he has to say and he'll have a week's worth of material, Letterman gets credit for saying what he "really thinks" and having controversial guests as opposed to the usual actor pushing a movie or whatever. I wouldn't be surprised if they agreed ahead of time that they would argue and what the main themes would be.
I actually like Letterman the most out of the big 3 late night hosts, more than Leno or Conan atleast.