CSLA .NET 1.x to 3.x supported Windows Forms directly through the base collection types. CSLA .NET 3.x partially supported WPF, but WPF requires a different collection base class for full support, so there's no way to fully support both Windows Forms and WPF.
CSLA .NET 4.0 will fully support WPF through the base collection types. This means the base collection types won't work well with Windows Forms. I plan to copy the 3.8 base collection types to the Csla.Windows namespace (or just rename them) so Windows Forms will continue to be fully supported - you just need to pick the correct base collection types depending on whether you are using Windows Forms or not.
CSLA .NET for Silverlight already uses the correct base class for the base collection types, so you can look there to see what CSLA .NET 4.0 will look like. The SL types offer almost complete backward compatibility, but work with XAML binding, not Windows Forms binding.