If you've ever had to create a sequence diagram from scratch, i.e., created all classes, interfaces, methods, etc, then you'll know it is a time consuming process and will appreciate the ability to reverse engineer existing code.  

Typically when I start a new contract the first thing I do is reverse engineer the code into a Visio document and use it as a template for diagrams I'll create (as I learn their system). Fortunately my last few contracts have had Visio and an MSDN license so I could grab the Visio for Architects application and get setup at  no client expense.

This may not be the case in the future....  I upgraded my current contract's environment to Visual Studio 2008, so that we could take advantage of the new Web, WCF and Smart Client Software Factories and was disappointed to find  that there was no way to reverse engineer the code to diagram our finished projects.  I am not alone in my disappointment (ref: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2488400&SiteID=1).

UPDATED - in the above link I provide a work-around to the problem; convert the solution and project(s) to Visual Studio 2005.

As a result my current client is in the process of purchasing the Altova UModel application - it has a reverse engineering capability for Visual Studio 2008, is reasonably priced and has a professional look and feel. 

As for me... I'm in the process of downloading Visual Studio 2005 from MSDN (my Vista development box only has VS2008) so that I can set up a new (different) development box with XP, VS2005 and Visio for Enterprise Architects.  The only purpose in life for this box will be to reverse engineer VS2005 source code into a Visio document (so I don't have to use my wife's laptop anymore;).  

Fortunately the Unity 1.1 download is a VS2005 project so I'll have no problems reverse engineering it to create diagrams as I learn how it works under the hood.  I'm dreading the work I'll have to do when it comes time to address the CompositeWPF (VS2008) diagrams - I'll manually create the objects before I'll switch....

In case you are like me and don't have the required applications on your development box I'm providing a reverse engineered Unity 1.1 Visio Document for your convenience. 

Unity1.1.vsd (514.50 kb)