The Popfly Game Engine is a C# Silverlight project which builds a game engine capable of consuming data files from the games formerly hosted on popfly.com.
This is a slight branch of the last production game engine on popfly.com which removes features dependent on the site like high scores.
“I wanted to keep using the same code as much as possible for both the standard Silverlight web app version of the project as well as the WP7S version. To do this, I created a second project “WindowsPhoneGameEngine” at the same level as the original “GameEngine” solution. I then added an additional conditional compilation symbol “Phone” to the WindowsPhoneGameEngine and use #if statements to deal with any required differences. This allowed the same files to compile in either project. I did run into a few snafus.”
Details: Ben Andersons MSDN Blog.