My boss started blogging (where he finds the time I have no idea), so figured I should put a plug in for him <g>: http://www.fernicola.org/loquitor/ . I think this makes me the first WPF blogger to have a blogging boss...

Robbie has also started blogging, he's the guy behind WPF's themes support and all-around good guy for how to make your application look its best: http://www.notstatic.com

I also recently found out the following non-Microsoft people have WPF-related blogs:

(OK, I've been reading Drew's for a while but the others I've only heard about recently.  Good stuff!)