Mike Harsh blogged about MLB.com rolling out video that runs on the Silverlight RC1 build. I think this is the first customer-facing use of Silverlight aside from a couple of stints on the Microsoft homepage. The MLB.com partnership was announced at MIX. I couldn't get it to work on my Mac with Camino but on my PC it looked good.
MLB.com looks like they've rolled out a whole video portal powered by Silverlight. They have a bunch of highlight footage as well as all the typical features such as being able to link to the video and send to a friend. The only thing I didn't notice was the ability to embed the videos somewhere.
As far as I can tell MLB.com is still using Flash for some of their videos. On the front page the videos that played were all done in Flash. They're also using the same skin on the Flash videos and the Silverlight videos so for anyone who has both of the plugins running the video experience should be fairly seamless (unless you're in my boat). I believe the announcement at MIX referred to MLB.tv premium, so the video rollout may be a test of the RC build before they convert the premium site over for the full release.