Steve Fox and Bill Sheldon have put together an example application that shows deep integration of the Office Client with SharePoint. Office Business Application (OBAs) such as this one address the gap between the idealized world where business processes follow a fixed procedure, and the real world, where processes are dynamic, collaborative, and much less structured. Users of this OBA work in their familiar Microsoft Office environment, while interacting with their line-of-business data.
This example shows customization of the Office Client Ribbon and Action Pane, custom Outlook Form Regions, integration with a SharePoint workflow, use of Open XML, and integration with Unified Communications.
You can find source code for this sample at http://www.codeplex.com/obasales. Steve and Bill have written an accompanying book, Professional Office Business Application Development: Using Microsoft Office SharePoint Server 2007 and VSTO, that explains how to build applications like these.
If you are building an OBA, this example application and book are invaluable resources.