Dual-deploying a WPF app natively and as a XBAP

How to create a WPF application that can be deployed as either a XBAP or as a native Windows application with as little overhead as possible? XBAPs are binary compatible with WPF Windows applications and run on top of the same CLR (Unlike Silverlight). They still differ in some ways.

From deployment point of view their largest difference is that a XBAP has a Page control as the main container while a Windows application has a Window control. It would be rather trivial to make the Windows application use the Page control but I feel (Feel free to disagree) that using a Page navigation method in a Windows application is rather unintuitive.

The second major difference is that XBAPs by default are executed in a partially trusted environment. This can be bypassed though and the XBAPs can be made run in full trust mode. The partial trust mode with XBAPs means you cannot access for example remote web services or native code from the XBAPs directly.

What would be an easy and maintainable way to achieve the following with the above issues?

Requirements

1. Deployable in both ways: as a XBAP over http and as a stand-alone WPF executable.
2. Shows information from a remote web server such as flickr or through an unmanaged API.
3. If possible, the deployment methods should remain faithful to their platforms. That is to say: Use Pages in XBAP where those make sense and dialogs in the Windows app where those make sense.

Restrictions

1. Platform optimised. The stuff required by the XBAP shouldn't affect the Windows client and vice versa.
2. The deployment options share as much code as possible. This helps in testing and maintenance.
3. Running the XBAP should be as simple as possible. Certificate prerequirement would defeat the purpose of a XBAP.

What I have gathered is that first requirement requires two separate projects. The second restriction could be worked upon by having a user control project which contains all the UI and is referenced by the two deployment projects. A WCF web service could work around the limitations of XBAPs.

If employing a solution similar to the above lines the first restriction would require some work as the Windows client could do without the WCF service as it's already running in full trust mode. Also the third requirement would need some deployment specific code so all the UI code cannot be in the user control project.

I'd like to know what solutions could be used to resolve these problems. Feel free to ignore the partial solution. It's there mainly to explain the point and to prevent the "Thought of this already" comments. So just to clarify, I'm interested in all solutions to this problem, not just those that abide to the partial solution.

The reason for interest to this is that a solution would mean there's no need to ponder between a WPF Windows app and a XBAP as these both could be created with little more effort and the client could choose whichever suits them best.

You may want to take a look at Prism (at codeplex) It helps to target Wpf, Xbap and Silverlight with minimum code changes (the amount of extra work depends on your project).

Here's how to use it to create Wpf and Xbap application: Prism and XBap Applications