Dart is a new web language from Google, and aims to solve a similar set of problems that GWT solves, but with the addition of a dedicated in-browser and server side Dart VM (which improves runtime performance greatly).
If you already code Java, and have to build web applications in a team environment, then you'll appreciate the tooling that you get from your IDE and static analysis tools. The Dart project brings this set of tooling to the building browser apps.
Dart in Action takes you from the simple "hello world" app, through functional and OO programming paradigms, and through to more advanced topics such as async programming and concurrency. You'll build a client-side, single-page web app, and hook that up to a Dart powered server, along the way making use of HTML5 features such as local storage, appcache and websockets.
I'm here all week to answer any Dart questions you have, whether about Dart in Action, or about the Dart language generally (please start new threads for specific questions).
Dart should be reaching version 1.0 this year, so now is a great time to get up to speed with this exciting and productive language.