To say that MIX day 1 was a big day for Silverlight is a bit of an understatement. You can watch the details of it yourself up here on the visit mix website;
and if you then want to follow up and grab Silverlight 4 bits then visit;
which will point you to lots of resources, including the bits for download and then take a read of John’s excellent whitepaper;
and Adam’s (equally excellent) paper on Expression Blend 4;
and if it’s Windows Phone 7 Series information you’re after then visit;
and catch the MIX sessions online yourself.