Maybe you faced the situation where you had code that was to be used on your server side services and also on your silverlight clients.
Having this situation required some tricks because Silverlight Class libraries could not be used on .NET projects and viceversa.
Common workarounds were to share files and use precompilation directive iack!!
Well VS 2011 has a concept called Portable Class Libraries which allow you to create class libraries that can be use in Windows Phone, Silverlight, .NET framework, etc.
And if you have VS 2010 you don't have to suffer. Just use the Portable Library Tools from from the VS Extensions and start sharing code (see this image form the VS Extensions site)
For more details about Portable Libraries check the MSDN documentation page: http://msdn.microsoft.com/en-us/library/gg597391.aspx