I've seen MS's demo of a healthcare app using Silverlight and it is gorgeous. I would say that without a doubt it is the future as it builds upon the experiences of WinForm and restructures things to get around the problems of WinForm.
That said, at present WinForm is obviously more mature and probably has a shorter time to market due to the stability of the platform and wealth of resources, tools and knowledge.
I would expect that WinForm apps will slowly peter out within another decade but for now it is still a very viable platform.
