Comment Write (nearly) once (Score 1) 197
At the moment the best alternative is the write the application in OpenGL ES or HTML5. The choice depends on the type of application. Both will need a small wrapper on each platform. PhoneGap is a good choice if you go with HTML5.
I'm not sure how viable the OpenGL ES option is for Windows Phone, but then I personally loathe that platform anyway.
As for look and feel consistency, look at Angry Birds to see that that is not needed at all.