Apns-Sharp
http://code.google.com/p/apns-sharp/
Apple iPhone Push Notifications Library for .NET/Mono written in C#

Plugins in C#
http://www.codeproject.com/KB/cs/pluginsincsharp.aspx
Article on Building Plugins in C# Using Reflection.  This is a very old article but may still be useful!

Plugins in C# 2.0: A Generics Enabled Extension Library
http://www.codeproject.com/KB/cs/ExtensionManagerLibrary.aspx
Library and Article for adding extensions/plugins to your own .NET/Mono app, based on Plugins in C# Article