Earlier this week Microsoft announced that the platform restriction on using portable class library reference assemblies on non-Windows platforms has been removed.
PCLs are a great invention. They allow us to create assemblies that can be used on multiple platforms. This means I can bundle up some code and refer to it from a Windows Phone app, a Windows 8 app, a website, a desktop app and more.
This means I can reuse code without having to test it on each platform.
PCLs are great. However there was one big issue. If I was also building an app for a non-Windows Platform such as Android or iOS using the Xamarin tools then, legally, I wasn't allowed to distribute those libraries. At best that was a bit of an inconvenience. Fortunately now this issue has gone away. You can now build libraries you can share with apps running on all platforms without that particular legal concern.
Note that this is on the newest release of the assemblies only so make sure you are using those when distributing to anything non-Windows.