Some users have reported an issue in which audio and video are out of sync after the update to iTunes 10.1. During movie playback, the audio suffers a delay in relation to the video (i.e. there is a gap between the image and the sound, or the video outpaces the audio).
An effective fix for this issue has not emerged thus far; as such, some users have resorted to downgrading to iTunes 10.0, via these instructions.