One of the things I've been intrigued by for a while now is the fact that code compiled for the .NET Compact Framework (all versions) executes very nicely on the full .NET Framework.
For example, my personal hobby project, "Jimmy Sudoku", is written in C# for the .NET Compact Framework 2.0. There are actually two install kits. The first is a .CAB file for Windows Mobile devices. The second is an .MSI for Windows 9x, XP, and Vista. The desktop install kit even serves two purposes. First, it installs the program on the desktop. Second, it leverages ActiveSync to push the .CAB up to the Windows Mobile device.
It's a .NET Compact Framework app especially for Windows Mobile devices, but many 'Jimmy' fans don't have a Windows Mobile device to run it on.
The coolest part is the ease in which all of the components inter-operate. The .EXE and .DLL's that are delivered to the mobile device are the very same as the ones that are delivered to the desktop. Like Silverlight to WPF, the Compact Framework is a compatible subset of the full framework, so interoperability is a given.
Even better, you can reference CF assemblies in Full framework assemblies. One immediate offshoot of this in my hobby project... the web service I built to service "Game of the Day" requests actually references the CF assembly that implements the game state model and game generator code. The assembly that generates games on Windows Mobile PDA's & cell phones is the very same assembly that generates games in the ASP.NET web service.
Admittedly, there are some bothersome differences between the CF and the Full .NET Framework. The CF does not support WPF. The CF has no facilities for printing. Also, while the CF does supports some of the common Windows Forms dialogs, it does not support File Save and File Open dialogs on Windows Mobile Standard Edition (Smart Phone / non-touchscreen) devices.
These differences can be overlooked to some extent, though, for the fact that one compiled assembly can execute on so many very different machine types. Further, with interoperability, one can extend a CF-based core code with full-framework support. For example, I'm currently playing with desktop print functionality for my hobby project.
Something that I'd really love to see, some day, is a good excuse to develop a Windows Forms app for a client that had shared components between the desktop and a mobile.
I can imagine that this model would be superb for a huge variety of applications, allowing a fully featured UI for the desktop version, and an excellent, 100% compatible, very low risk (almost "free") portable version.
I've often thought this would work great for apps that interface hardware, like:
vehicles of all sorts,
...simply plug in your PDA (via USB or Bluetooth), and it becomes a smart management device for the equipment, using the very same code that also runs on the desktop.