The night that NBC had flailingly fought to avoid arrives tonight Jay Leno going head-to-head against Conan O'Brien. Last year it worried that after allowing O'Brien to take over the Tonight show it would see Leno launch a competing show on another network -- probably ABC -- and see its late-night ratings drop. As things have now worked out, it is Leno who is back at the Tonight show and O'Brien challenging him on TBS. Oh, yes -- there's also another potent competitor fragmenting the late-night ratings Jon Stewart, who emerged last month with more 18-49-year-old viewers than Letterman, who landed in second place in that demo last week, and Leno, who placed third. Until Stewart's October victory, most TV pundits had assumed that O'Brien would be unable to draw anything close to the size of the audience for a broadcast network. Now, it's almost a given that, together, Stewart, O'Brien and, to a lesser degree Letterman, will suck up most of the younger viewers, who are also adept at using DVRs and computer software programs to record one or the other late-night show they miss. Today's (Monday) New York Times quoted an unidentified former late-night writer-producer as predicting that O'Brien will get a big first-night audience. "He'll get a little time to shine; maybe a bit less than everyone thinks. ... After a few nights, I think Conan will come down to his real level ... and that will have nothing to do with him. He's great. But you just can't get a whole lot of people to watch any one of these shows anymore."