Microsoft has released CTP 1 of codename ‘Acropolis’. Acropolis helps developers build modular .NET applications. It’s primary focus appears to be WPF, but it supports WinForms UI components as well.
These tools and components will ship as an add-on to Visual Studio 2008, and Brad Abrams says they will become part of Visual Studio and the .NET Framework in a future release beyond that.
You can download the bits here, and there are already several resources to help get you starting in building your own Acropolis-based UIs.
Go forth and build!