Since the recent announcement of Apple MobileMe -the MS ActiveSync like application- everybody in the Dev world have been speaking about what lies behind, about what gives it that desktop applications' smooth interface.. and here comes SproutCore.
MS offers Silverlight to deliver the WPF experience to the web but you need to install the client to all machines + it's not really "light" yet.
Adobe has the widely spread Flash Player but it stands against all modern SEO techniques as it can't be indexed by search crawlers -while Silverlight can be-
Apple's recently used it to develop their MobileMe service and they are also willing to optimize their next Safari to run SproutCore scripts even faster...
A demo on SproutCore site to show a UI built completely with the framework
SproutCore is nice but when you think that each site you build that uses SomeWordButtong.js will actually have to download this file to the user, it doesn't sound really effective unless the framework will be fully integrated into the browser which brings us back to a Silverlight comparison...
My personal opinion is, I prefer to build -What I call- Text-Based sites than having to install a plugin. This gives the ability for the browser to do some magic... isn't that what browsers made for?