Also porting is interesting - we're in a position where we have a bunch of highly optimized, debugged C/C++ 'backend' code. Since native code isn't supported on Windows 7 Phone, we'll have to port our app by rewriting it all in C#. Unfortunately, it's not practical option for us at the moment.
That does tend to be one of the biggest stumbling blocks for WP7. I'm surprised they haven't figured out a way to deal with that.
I'm going the other way - my own code is all in C# (Mac and Linux with Mono, and MonoTouch and MonoDroid for the mobile platforms) so deployment to any of the above platforms becomes relatively easy. (I'm writing games, so I can hoist a lot more of the otherwise platform-specific logic into OpenGL, OpenGL|ES, and XNA, respectively--still requires building different GUIs based on desktop vs. phone but is a lot nicer than Windows vs. Mac vs. Linux vs. iOS vs. Android.)
I wrote a GPS app (for offroad use) in December last year. It's a pretty poor app, and because it was crap I priced it high to discourage buyers. And now I'm getting mailed by Microsoft because they want to give me my money. It was quite amazing - I never tracked the app's performance, and ignored it, because I only wrote it to get familiar with the tools.
I've not written for any other platform, so have no comparison to offer.
