There is also a Learn WPF/E page up that acts as an intro FAQ, but then links off to the official WPF/E FAQ. Still, they did provide a nice browser compatibility matrix that shows that WPF/E supports IE/Firefox/Safari in Windows + Mac (except Firefox 2.0 on Mac, which will be supported by the next CTP).
WPF/E is a cross-platform / web subset of WPF, enabling XAML (an open specification) to be used in websites to create “rich experiences” (rich synergistic experiences?). The big draw is that you get to write in C#; Microsoft have created a cross-plaform mini-CLR.
Despite the similarities, Microsoft insist that they are not competing with Macromedia Flash, although I’m not sure how much I believe that.