WinForms is not going away anytime soon. The core under WPF still uses the same .NET windowing libraries, so it will be supported for the forseeable future.
WPF and XAML are an order of magnitude easier to develop for, especially for cool looking GUI's. The tool support is somewhat lacking but Visual Studio 2008 (Orcas) as well as the various Expression products are now in pretty much final stage and downloadable for free. You will have a much easier time building rich UI's with it than plain old WinForms, simply because it enables coding at a higher level of abstraction than before.
When you feel like porting your app to the mobile platform, it would be easy to convert it to to use Silverlight, which is the new codename for WPF/E, which is a lighter version of WPF meant for browser apps. It is supported on the Compact Framework (PocketPC, phones, mobile etc). When I say 'is available' though I don't mean right now
It should be out in the next release of CF.NET 3.5 (http://blogs.msdn.co...07/mix-and-medc.aspx