Mike Harsh discusses a bit about WPF/E as a cross platform solution, and includes a Firefox extension, leaving me to believe another Firefox extension from Microsoft is in the works (first being Microsoft Genuine Advantage). Mac support too.
I do wonder if yet another framework to fragment the computing world is needed. Especially with things like SVG, Canvas and the Web Applications 1.0 Specification aka (X)HTML5, among existing technologies already in motion. But that’s another discussion.