The ported game is their own, titled WordMonger, and was originally written in C# using OpenGL and MonoTouch. Converting it to the Zune HD with .NET was made possible by the XNA Game Studio 3.1 Zune HD extensions. While WordMonger will be available on the iPhone by Thanksgiving, the Zune HD version doesn't have a release date, as Microsoft isn't yet allowing third parties to publish their own apps on the Zune Marketplace.
There is a method of installing the game on your Zune HD, though, it requires a little elbow grease. You'll have to download and install Visual C#, .Net 3.5, and the XNA Game Studio 3.1. Then, you have to request the source code from Foundation42, and build/deploy it manually -- a little too much work for the Average Joe I'm guessing.