We're now nine weeks into the disappearance of's adults 18-49 lead over Letterman's and the two shows are still neck and neck in the 18-49 demo. Since late September/ early October, the shows have had exactly the same overall average (1.0) for adults 18-49 ratings.
This is close to looking like a permanent sea change. Not quite as dramatic as when Leno took the late night ratings lead from Letterman in the mid 90's and never gave it back, particularly because Letterman's unlikely to stay in his chair past 2012 when he'll be 65 and is rumored to be retiring, but a big change nevertheless.
The trend lines above are 4 week moving averages.
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Nielsen TV Ratings Data: ©2009 The Nielsen Company. All Rights Reserved.