The Blend RC has just been released. As an eager child I got hold of it and checked if my projects were looking good inside it!
Some of the things that I noticed right off the bat:
I can now see my custom Window control in all its glory right inside Blend. In the previous version I was only seeing some rectangles with solid colors.
Still having trouble with ResourceDictionaries that have been linked using absolute paths (eg: /Assets/PanelIcons.xaml). I know it works inside VS2005.
Storyboards that are not linked to any Triggers don’t show up in the Resources panel. I would love to have them editable.
I am sure there are more, but i’ll post them as I discover and probably post on the Blend newsgroups or Connect.