Nothing, as of August 2001. That's likely to change though.
Jon Udell "think[s] of WinForms as MFC (Microsoft Foundation Classes) for scripting." http://msdn.microsoft.com/vstudio/nextgen/technology/winforms.asp has Microsoft's own expression of the vision.
Note that Ximian has committed to including WinForms (and ADO, too) as part of its .NET implementation (more than Microsoft itself has done).