From a friend who works at NBC in New York…
What happened last night on SNL was that Ashlee Simpson got caught using pretaped vocals. Like you said, singers do pretape and then lip-sync on the air. Usually the show frowns on this but sometimes during rehearsal it becomes apparent that the performer needs it. I don't know the specifics here but I'm told Simpson had prerecorded tracks for both songs she did last night. Her first song was called "Pieces of Me" and it went fine. Second song was supposed to be a new one called "Autobiography" but someone screwed up and they started playing "Pieces of Me" again. Big oops. Someone did a fast patch for the West Coast feed to make it less obvious what had happened but it wasn't that the band started playing the wrong number. It was whoever was supposed to change the DAT or the CD or whatever her playback was on. I wasn't there at the time but I can imagine they rejected the idea of editing in the dress rehearsal tape for the West Coast since it would betray the show's "live" credo more than making a small audio edit. You're right that they'll probably edit the dress tape in when the show gets rerun unless the incident becomes too famous.
A question for anyone who knows: I seem to recall one or more occasions where the dress rehearsal version of some SNL sketch went so well that they didn't even try doing it again on the live broadcast. They just rolled tape from dress. Does anyone else remember this?
I felt bad for Ashlee Simpson, if only because all the late night comedians are going to switch the Britney Spears lip-syncing jokes over to her. These jokes have an interesting heritage. Before Britney, they were about Mariah Carey…then before her, the Backstreet Boys. And at one point, in a slightly different form, they were all about Milli Vanilli. Recently, Madonna has been accused of pre-taping vocals but Ashlee has probably beaten her out to claim the mantle. She will hold it until someone else gets caught allegedly mouthing lyrics. It'll probably be George W. Bush but maybe not before the election.