Here are some pretty comprehensive resources on the new features in our WPF 3.5 SP1 release.
Scott Guthrie's blog post on installation issues, features in Visual Studio and the .NET framework.
Jossef Goldberg's blog post on performance improvements in WPF.
Greg Schechter's blog post on new graphics features in WPF.
I'll be posting about some more new WPF features soon. The next one I'll post will be on the new formating capabilities so stay tuned.