A significant number of our ISV early adopters are interested in hosting the designer in their own applications. We now have a great article on just that http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnlong/html/WFDsgnRehst.asp.
The Windows SDK also contains two samples of this:
The trick is… to find the time to read all this stuff! 🙂