Through this blog post I just noticed that a new white paper on Windows Presentation Foundation ("Avalon") is up on the Vista Developer Center on MSDN.
The paper provides a very good overview of all the different features and aspects of WPF. It focuses more on the whys than the hows. Like why Microsoft has decided to build a new unified presentation platform to supersede GDI and DHTML.
On a side note: The homepage of the Vista Developer Center has an RSS feed, but strangely enough the last entry on the feed dates back to September 14, 2005. What's the point of having an RSS feed, when it's not up to date?! The MSDN Just Published RSS feed does have recent entries, but does not include the WPF white paper.