Check out the interview with Mike and Parimal:

http://www.ddj.com/documents/s=9776/ddj1134151231499/swigart3.htm

Excerpt:

MH: Sure. The statements you're making are exactly correct. The bottom line is, we're trying to learn from what we've done in the past, and from talking to our customers. And, even though some customers may say they want the migration approach, if you really dig into that, it's really hard to get that right. Our message to customers is that you need to be very conscious about moving to a new technology, and not think of it as some afterthought. You really don't want some little wizard making big decisions for you. You want to make good, strategic, decisions with your code base. That's one reason why we decided not to take the migration approach. In looking at this, we could have done some of the migration wizard stuff, but we just think it's the wrong approach.

With our approach, we think it makes more sense to let customers fully leverage their existing code base, and move forward to the WPF world at their own pace. What customers wouldn't want to hear is that they have to make a choice between Windows Forms and WPF; they would not want to be told that they have to rewrite the entire application to take advantage of any of WPF. The approach we're trying to take is, "You don't have to rewrite. You can move at your own pace, and decide what features and functions you want to keep in Windows Forms, and just use WPF in the areas of your application where it really makes sense."