Games are usually good candidates for portability because the GUI capabilities & conventions of the host OS need not affect what you do in your custom full screen rectangle.
This is usually true; your backend can be straight C and you can have a thin layer at the front end to glue your game to the target OS. This makes for easy porting between iOS and Android and Web OS.
But it won't currently work for WP7, as they don't allow anything but Managed C#. You have to do a top-to-bottom, from-scratch rewrite to target WP7.
I think there needs to be an easy way to port apps, then later sure try and differentiate yourself. The same way the Xbox works where the majority of games I can get on the PS3 also.