At this years NDC conference, I followed a couple of talks about mobile development. It was announced there that the people behind MonoTouch and MonoDroid were moving to a new company named Xamarin. Well, as of yesterday, they have actually made the move and all sources for doing cross platform development can be found at their site.
They are actually providing a really cool way of doing cross platform development. You don't need to learn a whole lot of Objective-C or Java to develop applications for iPhone or Android, but you can develop in C# .Net, the language you use for doing Windows Phone 7 development. The only thing you still need to provide, that is specific for the iPhone and Android devices, is the user interface specific for each device. If you develop your applications wisely, though, - that is, using one of the MV* patterns - this is a walk in the park.
For iPhone development you will need a MAC, because the iPhone SDK only works on iOS. You can then install Mono and MonoDevelop (instead of Visual Studio) for OSX to start developing. For people like me, who are used to tools like ReSharper, you will, however, again need to learn how to type.
For Android development, the Java SDK is needed, and again MonoDevelop or Visual Studio 2010. With some handy project linking in Visual Studio, you can easily provide a shared code base for all three platforms. On this, watch the NDC talk, by Jonas Follesoe, about cross platform development!
The good news also is, that Xamarin offers both MonoTouch and MonoDroid as a free trial version that doesn't expire. The only drawback to the trial is that you can't deploy to actual iPhone or Android devices, only to their respective emulators. But this should be enough to get you started. Once you want to try out specific features the emulators don't offer (like gps, camera, ...) you will need to move to a paid license (or once you want to deploy to the marketplace, that is).
You will however need a little bit of extra effort if your initial WP7 projects are hosted on TFS. MonoDevelop for now does not offer a build in way for synchronising projects with TFS. You can use Microsoft Visual Studio Team Explorer Everywhere 2010, which offers a command line utility for connecting to TFS. Also SvnBridge gives you an SVN kinda way for connecting to TFS. And also, you could move your code base to SVN or GitHub (I love that place, not only for its name) alltogether.
This weekend there is also the MonoSpace conference in Boston going on, specifically for Mono development. They have a couple of sessions on mobile development as well. I hope they will put videos on-line of their talks.
Since I don't own a MAC (yet), I will try out the MonoDroid SDK in the next couple of weeks. Be sure to check back in to watch the progress (and the walls I will probably hit).