Infragistics  is already deeply involved in Silverlight development . They are excited about the platform as the next-generation Web UI. They think there are several key wins in Silverlight  for Web application development and are exploring how they can best provide controls and tooling to complement what Microsoft provides in the box.
At TechEd this year the showed a proof-of-technology demo based on Silverlight 1.1. Controls they have been working on: Image Slider, Slider Panel and a Chart control.