Britney absolutely lipsync's most of the time. Everytime I've seen any performance by her, it's very obvious when they show her close-up that she's not singing.
And for Ashlee (I'm nothing without my sister's fame) Simpson, what did you really expect? For the 5 minutes I wasted one day watching her show and her recording her CD, she's absolutely horrible. She can't sing worth a damn. All I know is that her vocals are probably completely gone through with ProTools to clean up the fact that she can't sing. "My band started playing the wrong song." Yeah, mistakes can happen, so you jump in and sing, big deal. Guess she didn't have the confidence to do that. Hmmmmm.
