Miguel de Icaza, founder of the Mono project, says that support for Windows Presentation Foundation on Mono is possible, but would require funding for 15 to 20 developers over a period of two to three years. As an alternative he proposes using other toolkits, but they too need community support.
The first option he mentions is the use of Gtk# across Windows, OS X, and Linux.
For those who can afford to split their UI, Miguel recommends using the native toolkits for each platform. That means WPF on Windows, MonoMac on OS X, and Gtk# on Linux. Though he didn’t mention it, there is also MonoTouch for iPhone and MonoDroid for the Android platform. This would probably result in the best user experience.
A third option for those who can afford to ignore the mobile platforms is WinForms. Unfortunately Microsoft has essentially abandoned it and Mono is not actively working on it either.
Long term there are several more interesting options, but each of which would require significant community involvement to bring to fruition.
Monomac.Winforms: assist the effort to have a Winforms look-alike API that happens to be based entirely on MonoMac and provides a native experience at the expense of dropping compatibility with some Winforms features.
Create an SWT-like toolkit, like Eclipse did for Java, but this time for .NET. Mapping UI components to Cocoa, Gtk+ or WPF depending on the platform.
Use Silverlight on Windows. And then use a modified version of Moonlight on Linux (and assist porting Moonlight to Mac) to get enough support to integrate with the native OS (menus, file dialogs, file system access) and to access and embed OpenGL in their applications.