Just a quick note to say that the second in my three-part series of articles on accessing cabinet files from .NET is available at DevSource.  This article shows how to implement the callback functions and drive the Cabinet SDK functions.

Reading and Writing CAB Files from .NET, Part 1:  Creating a CABINET.DLL Interface

Reading and Writing CAB Files from .NET, Part 2: Using the CABINET.DLL Interface

You can find the code and Visual Studio project files here.