http://www.collegehumor.com/news/ashlee_snl.wmv From what I heard (I didn't see all of her first performance), she did what Britney Spears does - sang over a track of herself, but with her mic volume down reeeeeeeal low, and the backing track bumped up reeeeeal loud (which would account for the "echo" some people claim happened at the beginning of her first performance - she was singing at the same volume as the backing track).
As for the screw up, the vocals start out at the regular volume, and then whoever was working the track lowered it alot. However, if you listen, you can definitely still here it as her band is playing. Heh.
She was apparently supposed to sing her new single "Autobiography," which has a beginning very similar to Pieces of Me. I love how she blamed her band at the end of playing the wrong song. Gee, Ashlee, your voice started singing the wrong song, too! Even though the microphone was no where near your mouth...