The first thing to know about Acropolis is that it is not a new GUI toolkit. As it relies on Windows Presentation Foundation, everything it offers can already be done by .NET developers. What it offers is a higher-level abstraction for building applications that goes beyond the blank form developers are used to.
The Acropolis framework is going to be heavily tooled with wizards and deep integration into Visual Studio. The goal is to speed development by getting developers to focus more on business logic of an application and less on the overall GUI framework and look and feel issues.
Brad Abrams has an article demonstrating separation of concerns using Acropolis. In addition to showing two different GUI views, he also mentions that Acropolis can be used for non-WPF applications including those targeting the console.
By encouraging developers to naturally separate the business and presentation logic, it should be easier to develop unit tests against the business logic. The inability to effectively unit test user interfaces has long been a sticking point for those trying to use techniques like Test Driven Development.