You can develop for WP7 (Windows Phone 7) using either Silverlight 3 or XNA. I am currently using both for writing applications for applications / performance, and for the game.
Silverlight is a subset of Windows Presentation Foundation (WPF), and the knowledge and experience gained can easily be transferred to another. Other business-class applications (LOBs) are starting to use WPF / Silverlight every day.
I can see that Silverlight will be used in the coming years for several reasons: it is easier to deploy than ActiveX, it has excellent tool support (VS2010, Expression Blend), and you can use the existing code base. The current installation base for Silverlight is at 60% (sorry, there is no backup link, it was indicated in a telephone conference last week with Microsoft).
I receive calls / emails from recruitment agencies that often ask if I am interested in changing industries (from video games to commercial / government), as there is a huge demand for candidates who have WPF / Silverlight experience.
source share