The Reference Source Code Center team has just announced that the WPF 3.5 SP1 sources are now publicly available.  You can read about it here, http://blogs.msdn.com/rscc/archive/2008/10/22/wpf-sources-for-net-framework-3-5-sp1-are-available.aspx.  The post also has pointers on how to setup and where to post any issues or feedback.  Enjoy!