Getting IApplication

When working with IApplication, you can't call New AppRefClass() because AppRef is a singleton object.  Here's what you do instead. 

Type t2 = Type.GetTypeFromProgID("esriCore.AppRef");
System.Object obj = Activator.CreateInstance(t2);
IApplication app = obj as IApplication;

http://forums.esri.com/Thread.asp?c=93&f=1170&t=67573#177441