Microsoft has hastily released the February 2007 CTP of the Windows Presentation Foundation Everywhere runtime. The previous CTP expired prematurely.
WPF/E is the "light" version of WPF that will run in all major browsers across several platforms (including non-Windows platform). For instance, it also works in Mozilla Firefox. Think of it as the Microsoft competitor to Flash.
WPF/E is also good at displaying video with a light-weight video engine that does not depend on Windows Media Player. Again, comparable to Flash Video, but now using the WMV format.
I noticed that the speed of the WPF/E runtime has increased significantly when compared to the December 2006 CTP.
The WPF/E samples in the Channel 9 Playground have been updated. Go check them out if you are interested in WPF/E. If you don't have WPF/E installed, your browser should prompt you on how to install the runtime.