Erick Ellis and Mark Wilson-Thomas talk about what's new in the WPF editor for Visual Studio in .NET 3.5 SP1. Topics include faster performance both from WPF and optimizations of their own, new cool visualizers, the Document Outline (very useful for WPF Apps) and more.
And as Mark mentioned, if you want to get your feedback in for future updates, go to the Visual Studio WPF Designer Forum on MSDN.
Looking for more than tools? Check out the WPF 3.5 SP1 Overview video!