I am currently working on an appplication for iPhone and iPad. An application for Android phones and an application for Windows Phone 7.
One might imagine that changing between Objective C to Java to C# and Silverlight languages would be a mental challenge for an old chap like me. One could also assume that having a handle on the various API's that each system requires such that the aforementioned old chap could develop a well architected application on all three platforms would be a bit of a challenge too. The reality however is a bit different.
For all three platforms I use the same development kit, the same programming language and the same familiar set, or subset, of the API's I have been very comfortable with for the last ten years. You see, I use Visual Studio 2010, C# and .Net for everything.
I am able to create between 80 and 90 percent of the code using nonspecialised C# and the rest of the system, intractions with specialised APIs or the various graphics systems required by the different platforms. can be done using the Mono versions, MonoTouch or MonoDroid.
So, the conclusion for me at least is that Visual Studio 2010 is by far the most useful development system no matter what platform I am developing for.