Visual Studio Tools for Office 2005 and Microsoft Office 2007
When you install the VSTO 2005 one of the pre-requisites is Microsoft Office 2003 SP1.
There are several templates that allow you to create different kind of projects, from add-ins to document-level customizations.
If you are targeting Office 2007 the document-level customizations developed in VSTO2005 for Office 2003 will work. The only thing you can't do is take advantage of the new features in Office 2007.
Also, you can get the VSTO 2005 SE and get the templates to create add-ins specifically for Office 2007. But if you install Office 2007 on a machine with VSTO 2005 the document-level customizations templates will not work anymore. The reason is that those templates are looking for the Office 2003 PIAs (Primary Interop Assembly).
Document-level customization for Office 2007 is a expected feature in Visual Studio 'Orcas', expected to be release in Q4 2007.