"Southland" could easily top "Leno", but its leading off with episodes already aired on NBC so its unlikely.
I am anxious to see the season 2 episodes, but I don't know that I will re-watch season 1, maybe other than to continue to support the show.
Since "Sons of Anarchy" already beat Leno 18-49, I figure its only time before more cable shows share this feat. Mark your calendars, 2009, the official start of the end of broadcast TV as we know it.
Or maybe just the end of NBC as we have known it. The 5 day a week Leno move means they've already thrown in the towel. I'd guess that a similar ratings deterioration is a couple of years off for ABC - longer for CBS.