Portable Class Library for Xamarin Clients - Xamarin Component Store
Xamarin support IOS/Android/Windows Store/Windows Phone. A universal Service Stack client written using PCL would simplify code reuse. The code could be packaged up into a Xamarin Component Store download if necessary. Or be used in conjunction with MvvmCross for use in MVVM style coding and/or unit testing.

We’ve added PCL support for Android and iOS in NuGet packages, see this tutorial for using ServiceStack’s PCL libraries in Xamarin projects:
https://github.com/ServiceStack/Hello
See this guide for adding NuGet support to Xamarin Studio to be able to easily reference NuGet packages:
http://barambani.wordpress.com/2013/10/07/add-nuget-package-manager-and-servicestack-to-xamarin-studio-projects-2/
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Stuart Beattie commented
Do you have plans for a pcl client library that supports WP8.1, RT8.1 and Xamarin
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Dylan v.d Merwe commented