Announced at PDC this week
http://www.microsoft.com/downloads/details.aspx?FamilyID=6806e466-dd25-482b-a9b3-3f93d2599699&displaylang=en
Finally a version of blend that will open vs2010 projects (and of course handle Wpf4 and Silverlight 4)
It will run side by side with Blend 3
Enjoy 