In my opinion, the big pro of using one single environment (i.e. C#/.NET) is code portability. And cool things like LINQ that, once you get used to it, you can't live without. However, the few mobile OS's (iOS, Android, WP7) are quite different with regards to UI.
And, if I am not mistaken about your application, it's got a fair share of UI interactions if it is to run on a mobile device. Most mobile apps are like 80% UI code.
Therefore, you'll end up writing a separate set of UI code for each platform anyway -- for example, you'll be writing in Silverlight WP7 (and all the WPF goodness), you'll be writing a completely different set of code for iOS in Cocoa (IB, Views, controllers and stuff), you'll be writing yet a completely different set of code for Android.
My experience has always been that it take a lot of experience to write good UI code on any platform -- e.g. learning WPF/SL is already the nightmare that is, throw in Cocoa Touch and the whole Android mess. Of course you can write three sets of UI that look and feel reasonably similar, but chances are that you'll be trying so hard to reuse code and have common data structures that your UI's will end up sub-par when compared to dedicated apps -- and in this cut-throat world of mobile apps today, a non-super (not to mention sub-par) UI experience means death to your app.
Also, all three mobile environments have different connectivity paradigms, as well as multimedia paradigms. You end up writing three versions, and learning three environments, albeit writing in one language you're familiar with.
The most you're going to reuse is back-end modules. Decision engines, search routines, data-management etc. And even these are going to be problematic because you'll be force to have compromises in your data structures just to enable easy integration with three different sets of UI code working on three different UI paradigms. For example, do you use DependencyObjects for use to bind to Silverlight views in an MVVM model? If you do, it won't work with Cocoa's MVC model, and you have to code those bindings separately.
And since not all mobile environments enable you to use the full set of functionalities -- for example, MonoTouch for iOS does not generic constructs that cannot be determined at compile time. You're essentially using a very small subset of .NET (and must constant be reminding yourself what functionality can be used where) just so that you can run them all on three different platforms without significant changes.
Now image having all these limitations when you are writing for the WP7 platform, which supports the entire .NET features set. I don't know about you, but I'll go crazy. And your WP7 app is never going to be even close to being competitive with other apps out there.
In my opinion, the pain and the compromises are not worth it. You'll end up with three so-so apps, which people in neither of the platforms are going to like.
Unless all the goodness lies in your app's back-end logic, and it is so good that people are going to ignore UI issues just to get to your app's back-end functionalities. In my experience, this almost never happens.
