software written, for iOS or Android using Mono definitely would not run on Windows Phone 7. Any generic libraries and business objects that do not use platform specific APIs may be sharable, but that is all.
I'd venture that for a well-written, non-trivial app, most of the code will be in the 'generic libraries and business objects' -- meaning that porting an app to Android or iOS (if you're using MonoTouch) shouldn't be a ton of work.
It properly won't be enough just to put the binary there, but I do believe you over estimate the cost to porting your software, especially if porting is taken into account before you start to write it.
