LASTEST VERSION OF THE CUSTOM UI EDITOR - updated to work with the RTM build of Office 2007 and Beta build of Office 2010 - ATTACHED TO THIS POST
The 2007 Microsoft Office System has an all-new "ribbon" interface which developers can customize in many ways. One of the ways to extend the ribbon is to insert an XML part into an Office Open XML document that defines your custom UI. With this approach, you can add new groups and controls, hide existing ribbon elements, and define callbacks to handlers that you write in managed code.
The XML part is not part of the Open XML format itself but could be added to any XML document by using a new namespace. The Open XML file formats are fully extensible, so applications can store information like workflow, metadata, custom XML, or even custom UI behaviors in a document that aren't necessarily tied to the document content.
Adding your custom-UI part to an Office Open XML document requires that you create a folder within the package, drop in your part, then define a relationship to the new part. For more information about how it all works, check out this technical artice or the "Extending the Office 2007 UI with a Custom Ribbon" screencast on this page.
Attached to this post is a great utility for working with ribbon customizations stored in Office Open XML documents: Trang Luu's custom UI editor. It's a Windows tool that automates the process of putting your custom UI part into the package and defining a relationship to it: that's all handled by a single mouse click.
You open an existing document with File/Open, then enter your custom UI XML into the main edit box. (Click the image to the right for a full screen shot.) Other options include inserting icons for your custom control, generating callbacks, and validating your custom XML.
Install the tool and try it out, and if you have any questions about using it you can post them right here.