There's a groovy diagram of the Office 2007 System Logical Architecture available here.

The only thing I'd change is I'd make the InfoPath box on the left contain "Visual Studio Tools for Applications" as another tool in your Office 2007 toolkit.

Another thing that always confuses people is that VSTO 2005 SE only has support for building application level add-ins for Office 2007.  To build document level solutions for Word and Excel in Office 2007, you'll have to wait for the Orcas release of Visual Studio.  But it will be worth waiting for--I'll start blogging in the coming weeks about some of the cool new features for Office 2007 development in Visual Studio Orcas as the Beta 1 of Orcas is coming up and you'll be able to start using the new features.