I\'m working on an application we made WPF instead of Silverlight as we wanted a full blown desktop application with the whole unique feeling and advantages that gives. Howe
Keep in mind that Silverlight 4 is currently in Beta with no end-user client runtime available. Silverlight 4 shipped in April of 2010. If you are already developing an app in WPF for the desktop, then it's probably the right solution. However, it would be wise to keep your ear to the ground and follow what's happening in Silverlight in case you may eventually want to port your app to the web space or develop a different app in the web space.
Silverlight 4 brings Silverlight to a whole new level. Check out Tim Heuer's blog post for a lot of the new features. Also, see if you can find a video of the facebook app from the keynote when Silverlight 4 features were announced. That app highlights a ton of the new features that are desktop-focused.