We've made the WPF sources for .NET Framework 3.5 SP1 available today through download of Visual Studio 2008. Here is a list of available components:  

List of available assemblies

NPWPF.dll

ReachFramework.dll

PenImc.dll

System.Printing.dll

PresentationBuildTasks.dll

System.Windows.Presentation.dll

PresentationCFFRasterizer.dll

UIAutomationClient.dll

PresentationCFFRasterizerNative_v0300.dll

UIAutomationClientsideProviders.dll

PresentationCore.dll

UIAutomationProvider.dll

PresentationFramework.Aero.dll

UIAutomationTypes.dll

PresentationFramework.Classic.dll

WindowsBase.dll

PresentationFramework.Luna.dll

WindowsFormsIntegration.dll

PresentationFramework.dll

wpfgfx_v0300.dll

PresentationFramework.Royale.dll

PresentationFontCache.exe

PresentationHostDll.dll

PresentationHost.exe

PresentationHostProxy.dll

XamlViewer_v0300.exe

PresentationNative_v0300.dll

XPSViewer.exe

PresentationUI.dll

  

To read more details about the release, how to get started, and where to report any issue, please take a look at the Reference Source Code Center Team Blog.