It has been announced that Silverlight 4 launch keynote will be available on demand here, but currently there are only follow-up talks on Channel 9 (very interesting as well). They will probably fix this when they wake up, but for now, you can find the keynote here.
To save you a click, I’ve embedded the video here – it last for about an hour.
PS. Embedding doesn’t work right now, but I’ll leave the code in case they fix it. In the meantime, watch it here.