The third major update of WPF has just been released as a Beta, as part of the .Net Framework 3.5 Service Pack 1 Beta. Here's a few links to blogs that have more detail:

Scott’s Blog [VP]

http://weblogs.asp.net/scottgu/archive/2008/05/12/visual-studio-2008-and-net-framework-3-5-service-pack-1-beta.aspx

Soma’s Blog [VP]

http://blogs.msdn.com/somasegar/

Tim Sneath’s Blog [Dev Evangelist]

http://blogs.msdn.com/tims

Greg Schecter’s Blog [Architect]

http://blogs.msdn.com/greg_schechter/archive/2008/05/12/a-series-on-gpu-based-effects-for-wpf.aspx

Jossef Goldberg's Blog [PM] (more detailed breakdown of features per area)

http://blogs.msdn.com/jgoldb/archive/2008/05/15/what-s-new-for-performance-in-wpf-in-net-3-5-sp1.aspx

Vincent Sibal's Blog [SDET]

http://blogs.msdn.com/vinsibal/archive/tags/WPF+3.5+SP1/default.aspx

Channel 9 WPF Intro on SP1

http://channel9.msdn.com