What is a UML design tool, such as IBM Rational Software Architect, that can generate C# code from the UML design diagrams?

You could also use Visual Paradigm (http://www.visual-paradigm.com/). I used the modeling pack in Visual Studio 2010 and I found it not enough powerful. However, it does a great job if you only need a skeleton.

There are serveral good tools in this space, here's a couple that i have used and would recommend Enterprise Architect from Sparx Systems (http://www.sparxsystems.com/products/ea/8/index.html) UModel from Altova (http://www.altova.com/umodel.html) There are also open source options that are capable as well.

I know the team that wrote C# code generator, it was a pretty strong team. I think C# code generator does a pretty solid job. However, if you were to just generate skeleton code once and then move on, you could use almost any decent tool; you don't need Rational Software Architect (RSA) if model is not going to be live artifact beyond initial design. The real power of RSA is not when you use it as sketching and code skeleton generation tool, it is when you either do Model Driven Development (MDD), or when model and code evolve over time, when you need to auto-sync/auto-generate, refactor model/code, compare and merge models, etc. etc., I bet no modeling tool under the sun can match RSA power as of today. In my opinion, main drawback of RSA is that it is a desktop tool and most of the world is moving to cloud. RSA Design Manager (DM) is their cloud offering, which I don't find that impressive. They haven't gotten cloud correctly, and again IMHO, it is because they wanted RSA to evolve into DM instead of starting from a clean slate. That reduced architectural/design choices. RSA was hugely successful, and sometime success is its own biggest enemy.