Microsoft has created a page dedicated to Office Business Applications, or OBAs. There is also a starter kit with the following resources:

• OBA/VSTO Starter Kit v. 1.0 Whitepaper: http://msdn2.microsoft.com/en-us/library/bb498190.aspx
• OBA/VSTO Starter Kit v. 1.0 Installation Document:  http://msdn2.microsoft.com/en-us/library/bb498189.aspx  
• OBA/VSTO Starter Kit v. 1.0 Source Code: http://www.microsoft.com/downloads/details.aspx?FamilyID=120AAAA8-0A0C-4A0B-B605-4E08FFE7A786&displaylang=en

Good stuff if you are an ISV and want to build Office applications.