Here's the definitive list:

List of changes and fixed issues for Visual Studio 2008 Service Pack 1 Beta for Windows Presentation Foundation Designer

New features

• The Properties window now contains the Events tab. The Events tab lets you create events, assign events, and review events.
• The Properties window now includes a category sort option and an alphabetical sort option to allow for faster property location.
• Code changes have been made to the XAML Refactor/Rename definition and to the Go to definition. These changes allow XAML rename operations to occur automatically. Additionally, you can navigate the XAML definition by pressing F12.
• You can now drag controls or create controls from the toolbox in XAML view or in Design view. You can do this even if you use a split view configuration.
• Snaplines are now implemented for control margins. This lets the designer control a fixed distance from other controls, from container edges, or from gridlines.
• Tab controls now support TabItem activation and TabItem design. To do this, click the tab that you want to design.
• The Expander control now expands conditionally based on what is selected. You can design the contents of the Expander control at design time with affecting the IsExpanded attribute of the runtime.
• Many stability improvements have been added to Visual Studio 2008 SP1. These include improvements to document loading in the designer and to error reporting. Because of these improvements, you will be able to load more documents in the designer.