For the developer:
The most up-to-date book on WPF, XAML and how it relates to the .NET platform. As a bonus, Charles Petzold is a wonderful writer. I haven't read alot of programming books (and I'm still working my way through this one) but it's a remarkable read.
For the designer:
A blog on WPF by Microsoft Technical Evangelist Karsten Januszewski with some wonderful examples of what WPF is capable of.
Musings of a Windows Vista Technical Evangelist
Tim Sneath's WPF blog where he also links to a more definitive list of blogs on WPF.
The WPF Blog
A blog by Lee Brimelow of Frog Design, he keeps us honest, and excited about what you can do with WPF.
Channel 9 has some wonderful videos that demonstrate what's possible with WPF. The link above will take you content tagged with WPF.
This just scratches the surface of WPF and there are many more resources that all of these sites will take you to.