This is a great document - congratulations, guys.
A couple of comments... In the section on "Font Support" you say that Portable User Interface is an alias for Lucida Sans. Actually I think it is an alias for Lucida Grande. In the same section you also say that PIF doesn't support font styles.
I think this used to be true for Silverlight 1.0 but it isn't true for Silverlight 2.0 now. You can apply italics, bold and extra bold to PIF and it renders exactly as it does for Lucida Grande.
A very valuable resource though. Thanks.