Lost in this month's NBC latenight crisis coverage has been the rising power of cable in late night among young adults, particularly the success of Adult Swim.
The latenight sister to Cartoon Network, Adult Swim has averaged a 0.9 rating among adults 18-49 in the 11:30 p.m. hour season-to-date. That's just a tenth of a ratings point lower than O'Brien, Letterman and ABC's "Nightline," all of which are delivered a 1.0 rating in the demo in the fourth quarter of 2009.
Update: Commenter U.V. notes that it's mostly Family Guy reruns at 11:30p, and those reruns always seem to rank ahead of everything else. For the week ending 1/17, the best 11:30p airing of Family Guy averaged a 1.4 adults 18-49 rating between 11:30p-12a. It could be that Family Guy is beating Letterman in the 18-49 demographic, but unfortunately we don't have half hour data for "Late Show" (or "Tonight Show") to make those comparisons.