If you've attempted to host the workflow designer in .net <= 3.5 you'll remember it's not such an easy achievement.
However doing so in wpf .net 4.0 is quite trivial.
Have a look at the Visual Studio 2010 .NET 4 training kit and you'll see for your self.
Here's my first appempt this evening that took all of 10 minutes.
Make sure to download the training kit even if you are an experienced developer, it's got some gems in it.