Silverlight is a topic I know next to nothing about, but I am seeing some interesting stuff the flash video stack doesn't really have..

They're introducing adaptive streaming.. Allowing videostreams to automatically adjust the bitrate depending on the bandwidth available. Flash has the concept of D-frames, but hardly anyone uses it and generally with less bandwidth you just end up with framedrops..

IIS7 Media pack can be configured to have bitrate throttling, useful for progressive downloads.. This is more an II7 feature than a silverlight feature though..

Thats where it ended, maybe there's more new features but the presentation was short..