I think you're looking for the MonoMac project. It's doubtful that you want to develop actual UI using WPF. Binding to native widget will give your app a much more integrated feel.
Sorry if I've misinterpreted your question as I'm not big on .NET but there are full, commercial OS X apps developed in C# on Mono: http://thread.gmane.org/gmane.comp.gnome.mono.devel/35827 .. an interesting hackaround to get a "native" interface though.
http://www.mono-project.com/MonoMac