Really exciting news today at PDC - Scott Guthrie announced Silverlight 4 Beta bits are now
available for download.
Another client-oriented release that contains some of the most wanted features like printing, mic and video support, drag and drop, rich text, RTL text, etc. The list continues on and on, you can read about it here.
From developer's perspective there is enhanced data binding support (StringFormat, IDataErrorInfo, TargetNullValue, etc), implicit styles (type based style theming), commanding, etc. For out of browser applications now there is HTML hosting support, Windowing API, notifications, elevated privileges support, etc
More on all those features will follow in corresponding articles.