My name is Aaron, and I’m a PM for the Messenger Voice, Video and Sounds team. We recently released an update to our earlier 8.5 Beta (which Jason pointed out earlier) that has many improvements to video call, especially on Vista. If you’ve occasionally run into audio jitter or noise in a Video Call, our latest version should help. We also improved echo support and a small issue with the volume sliders.
There are other updates as well, so head on over to http://get.live.com/betas/ and try the latest version.