I saw an article in Variety awhile back (can't find the link now) speculating about the possible shrinking of prime time - making it 9-11, or 8-10 (instead of the 3 hour slot it occupies now). Well - this news might be related, or it might not: Jay Leno is staying on NBC, and effectively moving his show to 10PM:

The new show, which will begin next fall, is expected to be set in Mr. Leno's longtime studio in Burbank, Calif. Mr. Leno is expected to retain many of the most popular elements of his "Tonight Show," including his monologue and bits like "Headlines" and "Jay Walking." One "Tonight Show" staff member said the new program would not be a variety show.

So does that more or less shrink prime time on NBC to 8-10, and toss the "Late Show" back to 10 PM? Sure, Conan will still be on at 11, but I don't think he has the following that Leno has. It's an interesting move, because Leno's show will be vastly cheaper to run that just about anything else they could put on at 10 PM. Makes me go Hmm.