If you're planning on writing a family of class libraries that will support multiple .NET frameworks (and/or platforms), you may want to create a "Portable Class Library".
Full details are in the blog post Targeting Multiple Platforms with Portable Code (Overview). In Visual Studio 2012, you simply create a new "Portable Class Library" project, whereas in Visual Studio 2010 you'll need to install the Portable Library Tools first.
With a PCL project, you can target the following platforms:
- .NET Framework 4, 4.0.3, and 4.5
- .NET for Metro style apps (which I presume includes Windows Phone 8 and Windows 8 RT)
- Windows Phone 7.x
- Silverlight 4 and 5
- Xbox 360
However, note that .NET Framework 3.5 is not currently included in this list. The Portable Library Tools were written so that other platforms can be added in the future, with Mono support currently at the top of the list.